Cmd-TAB is not working in 10.9

Works fine for me. Consider reinstalling:

Reinstall Lion, Mountain Lion, or Mavericks without erasing drive

Boot to the Recovery HD:

Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.

Repair

When the recovery menu appears select Disk Utility. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported then click on the Repair Permissions button. When the process is completed, then quit DU and return to the main menu. Select Restart from the Apple menu.

Reinstall Mountain Lion or Mavericks

OS X Mavericks- Reinstall OS X

OS X Mountain Lion- Reinstall OS X

OS X Lion- Reinstall Mac OS X

Note: You will need an active Internet connection. I suggest using Ethernet

if possible because it is three times faster than wireless.

Please read this whole message before doing anything.

This procedure is a test, not a solution. Don’t be disappointed when you find that nothing has changed after you complete it.

Step 1

The purpose of this step is to determine whether the problem is localized to your user account.

Enable guest logins* and log in as Guest. Don't use the Safari-only “Guest User” login created by “Find My Mac.”

While logged in as Guest, you won’t have access to any of your personal files or settings. Applications will behave as if you were running them for the first time. Don’t be alarmed by this; it’s normal. If you need any passwords or other personal data in order to complete the test, memorize, print, or write them down before you begin.

Test while logged in as Guest. Same problem?

After testing, log out of the guest account and, in your own account, disable it if you wish. Any files you created in the guest account will be deleted automatically when you log out of it.

*Note: If you’ve activated “Find My Mac” or FileVault, then you can’t enable the Guest account. The “Guest User” login created by “Find My Mac” is not the same. Create a new account in which to test, and delete it, including its home folder, after testing.

Step 2

The purpose of this step is to determine whether the problem is caused by third-party system modifications that load automatically at startup or login, by a peripheral device, by a font conflict, or by corruption of the file system or of certain system caches.

Disconnect all wired peripherals except those needed for the test, and remove all aftermarket expansion cards, if applicable. Start up in safe mode and log in to the account with the problem. You must hold down the shift key twice: once when you boot, and again when you log in.

Note: If FileVault is enabled, or if a firmware password is set, or if the boot volume is a software RAID, you can’t do this. Ask for further instructions.

Safe mode is much slower to boot and run than normal, with limited graphics performance, and some things won’t work at all, including sound output and Wi-Fi on certain models. The next normal boot may also be somewhat slow.

The login screen appears even if you usually log in automatically. You must know your login password in order to log in. If you’ve forgotten the password, you will need to reset it before you begin.

Test while in safe mode. Same problem?

After testing, reboot as usual (not in safe mode) and verify that you still have the problem. Post the results of Steps 1 and 2.

Here's a summary of what you need to do: Copy a line of text from this web page into the window of another application. Wait for the script to run. It usually takes a couple of minutes. Then paste the results, which will have been copied automatically, back into a reply on this page. The sequence is: copy, paste, wait, paste again. Details follow.

4. You may have started the computer in "safe" mode. Preferably, these steps should be taken in “normal” mode. If the system is now in safe mode and works well enough in normal mode to run the test, restart as usual. If you can only test in safe mode, do that.

5. If you have more than one user, and the one affected by the problem is not an administrator, then please run the test twice: once while logged in as the affected user, and once as an administrator. The results may be different. The user that is created automatically on a new computer when you start it for the first time is an administrator. If you can't log in as an administrator, test as the affected user. Most personal Macs have only one user, and in that case this section doesn’t apply.

6. The script is a single long line, all of which must be selected. You can accomplish this easily by triple-clicking anywhere in the line. The whole line will highlight, though you may not see all of it in your browser, and you can then copy it. If you try to select the line by dragging across the part you can see, you won't get all of it.

Triple-click anywhere in the line of text below on this page to select it:

PATH=/usr/bin:/bin:/usr/sbin:/sbin; clear; Fb='%s\n\t(%s)\n'; Fm='\n%s:\n\n%s\n'; Fr='\nRAM details:\n%s\n'; Fs='\n%s: %s\n'; Fu='user %s%%, system %s%%'; PB="/usr/libexec/PlistBuddy -c Print"; A () { [[ a -eq 0 ]]; }; M () { find -L "$d" -type f | while read f; do file -b "$f" | egrep -q XML\|exec && echo $f; done; }; Pc () { o=`grep -v '^ *#' "$2"`; Pm "$1"; }; Pm () { [[ "$o" ]] && o=`sed '/^ *$/d; s/^ */ /' <<< "$o"` && printf "$Fm" "$1" "$o"; }; Pp () { o=`$PB "$2" | awk -F'= ' \/$3'/{print $2}'`; Pm "$1"; }; Ps () { o=`echo $o`; [[ ! "$o" =~ ^0?$ ]] && printf "$Fs" "$1" "$o"; }; R () { o=; [[ r -eq 0 ]]; }; SP () { system_profiler SP${1}DataType; }; id | grep -qw '80(admin)'; a=$?; A && sudo true; r=$?; t=`date +%s`; clear; { A || echo $'No admin access\n'; A && ! R && echo $'No root access\n'; SP Software | sed '8!d;s/^ *//'; o=`SP Hardware | awk '/Mem/{print $2}'`; o=$((o<4?o:0)); Ps "Total RAM (GB)"; o=`SP Memory | sed '1,5d; /[my].*:/d'`; [[ "$o" =~ s:\ [^O]|x([^08]||0[^2]8[^0]) ]] && printf "$Fr" "$o"; o=`SP Diagnostics | sed '5,6!d'`; [[ "$o" =~ Pass ]] || Pm "POST"; o=`SP USB | sed -n '1d; /:$/{s/ *:$//;x;s/\n//p;}; /V.*[(]/{s/^[^(]*/ /;H;}; /Apple/{s/.//g;h;}'`; Pm "USB"; o=`pmset -g therm | sed 's/^.*C/C/'`; [[ "$o" =~ No\ th|pms ]] && o=; Pm "Thermal conditions"; o=`pmset -g sysload | grep -v :`; [[ "$o" =~ =\ [^GO] ]] || o=; Pm "System load advisory"; o=`nvram boot-args | awk '{$1=""; print}'`; Ps "boot-args"; d=(/ ""); D=(System User); for i in 0 1; do o=`cd ${d[$i]}L*/L*/Dia* && ls | awk -F_ '!/ag$/{$NF=a[split($NF,a,".")]; print}' | tail`; Pm "${D[$i]} diagnostics"; done; o=`syslog -F bsd -k Sender kernel -k Message CReq 'GPU |hfs: Ru|I/O e|n Cause: -|NVDA\(|pagin|SATA W|ssert|timed? ?o' | tail -n25 | awk '/:/{$4=""; $5=""};1'`; Pm "Kernel messages"; o=`df -m / | awk 'NR==2 {print $4}'`; o=$((o<5120?o:0)); Ps "Free space (MiB)"; o=$(($(vm_stat | awk '/eo/{sub("\\.",""); print $2}')/256)); o=$((o>=1024?o:0)); Ps "Pageouts (MiB)"; s=( `sar -u 1 10 | sed '$!d'` ); [[ s[4] -lt 85 ]] && o=`printf "$Fu" ${s[1]} ${s[3]}` || o=; Ps "Total CPU usage" && { s=(`ps acrx -o comm,ruid,%cpu | sed '2!d'`); o=${s[2]}%; Ps "CPU usage by process \"$s\" with UID ${s[1]}"; }; s=(`top -R -l1 -n1 -o prt -stats command,uid,prt | sed '$!d'`); s[2]=${s[2]%[+-]}; o=$((s[2]>=25000?s[2]:0)); Ps "Mach ports used by process \"$s\" with UID ${s[1]}"; o=`kextstat -kl | grep -v com\\.apple | cut -c53- | cut -d\< -f1`; Pm "Loaded extrinsic kernel extensions"; R && o=`sudo launchctl list | sed 1d | awk '!/0x|com\.(apple|openssh|vix\.cron)|org\.(amav|apac|calendarse|cups|dove|isc|ntp|post[fg]|x)/{print $3}'`; Pm "Extrinsic system jobs"; o=`launchctl list | sed 1d | awk '!/0x|com\.apple|org\.(x|openbsd)|\.[0-9]+$/{print $3}'`; Pm "Extrinsic agents"; o=`for d in {/,}L*/Lau*; do M; done | grep -v com\.apple\.CSConfig | while read f; do ID=$($PB\ :Label "$f") || ID="No job label"; printf "$Fb" "$f" "$ID"; done`; Pm "launchd items"; o=`for d in /{S*/,}L*/Star*; do M; done`; Pm "Startup items"; o=`find -L /S*/L*/E* {/,}L*/{A*d,Compon,Ex,In,Keyb,Mail/B,P*P,Qu*T,Scripti,Servi,Spo}* -type d -name Contents -prune | while read d; do ID=$($PB\ :CFBundleIdentifier "$d/Info.plist") || ID="No bundle ID"; [[ "$ID" =~ ^com\.apple\.[^x]|Accusys|ArcMSR|ATTO|HDPro|HighPoint|driver\.stex|hp-fax|\.hpio|JMicron|microsoft\.MDI|print|SoftRAID ]] || printf "$Fb" "${d%/Contents}" "$ID"; done`; Pm "Extrinsic loadable bundles"; o=`find -L /u*/{,*/}lib -type f | while read f; do file -b "$f" | grep -qw shared && ! codesign -v "$f" && echo $f; done`; Pm "Unsigned shared libraries"; o=`launchctl getenv DYLD_INSERT_LIBRARIES`; Pm "Inserted libraries"; o=`find -L {,/u*/lo*}/e*/periodic -type f -mtime -10d`; Pm "Modified periodic scripts"; o=`scutil --proxy | grep Prox`; Pm "Proxies"; o=`scutil --dns | awk '/r\[0\] /{if ($NF !~ /^1(0|72\.(1[6-9]|2[0-9]|3[0-1])|92\.168)\./) print $NF; exit}'`; Ps "DNS"; R && o=`sudo profiles -P | grep : | wc -l`; Ps "Profiles"; f=auto_master; [[ `md5 -q /etc/$f` =~ ^b166 ]] || Pc $f /etc/$f; for f in fstab sysctl.conf crontab launchd.conf; do Pc $f /etc/$f; done; Pc "hosts" <(grep -v 'host *$' /etc/hosts); Pc "User launchd" ~/.launchd*; R && Pc "Root crontab" <(sudo crontab -l); Pc "User crontab" <(crontab -l); R && o=`sudo defaults read com.apple.loginwindow LoginHook`; Pm "Login hook"; Pp "Global login items" /L*/P*/loginw* Path; Pp "User login items" L*/P*/*loginit* Name; Pp "Safari extensions" L*/Saf*/*/E*.plist Bundle | sed 's/\..*$//;s/-[1-9]$//'; o=`find ~ $TMPDIR.. \( -flags +sappnd,schg,uappnd,uchg -o ! -user $UID -o ! -perm -600 \) | wc -l`; Ps "Restricted user files"; cd; o=`SP Fonts | egrep "Valid: N|Duplicate: Y" | wc -l`; Ps "Font problems"; o=`find L*/{Con,Pref}* -type f ! -size 0 -name *.plist | while read f; do plutil -s "$f" >&- || echo $f; done`; Pm "Bad plists"; d=(Desktop L*/Keyc*); n=(20 7); for i in 0 1; do o=`find "${d[$i]}" -type f -maxdepth 1 | wc -l`; o=$((o<=n[$i]?0:o)); Ps "${d[$i]##*/} file count"; done; o=$((`date +%s`-t)); Ps "Elapsed time (s)"; } 2>/dev/null | pbcopy; exit 2>&-

exec sh

If you're logged in as an administrator, you'll be prompted for your login password. Nothing will be displayed when you type it. You will not see the usual dots in place of typed characters. Make sure caps lock is off. Type carefully and then press return. You may get a one-time warning to be careful. If you make three failed attempts to enter the password, the test will run anyway, but it will produce less information. In most cases, the difference is not important. If you don't know your password, or if you prefer not to enter it, just press return three times at the password prompt.

If you're not logged in as an administrator, you won't be prompted for a password. The test will still run. It just won't do anything that takes administrator privileges.

The test may take up to a few minutes to run, depending on how many files you have and the speed of the computer. A computer that's abnormally slow may take longer to run the test. While it's running, there will be nothing in the Terminal window and no indication of progress. Wait for the line "[Process completed]" to appear. If you don't see it within half an hour or so, the test probably won't complete in a reasonable time. In that case, close the Terminal window and report your results. No harm will be done.

8. When the test is complete, quit Terminal. The results will have been copied to the Clipboard automatically. They are not shown in the Terminal window. Please don't copy anything from there. All you have to do is start a reply to this comment and then paste by pressing command-V again.

If any private information, such as your name or email address, appears in the results, anonymize it before posting. Usually that won't be necessary.

When you post the results, you might see the message, "You have included content in your post that is not permitted." It means that the forum software has misidentified something in the post as a violation of the rules. If it happens, please post the test results on Pastebin, then post a link here to the page you created.

Note: This is a public forum, and others may give you advice based on the results of the test. They speak only for themselves, and I don't necessarily agree with them.

________________________________

Sometimes the forum software corrupts the script. Please try again, and make sure you're copying the whole line. If it still doesn't work, try another web browser; e.g., Firefox, if you're using Safari. Don't copy the script from an email notification.

Then we will have to do this the hard way. First, what are your login items in the Users & Groups preference pane?

Did you ever test in safe mode, and if so, what was the result?

There could be several reasons for it. If you can't boot in safe mode, you can still log in in safe mode by holding down the shift key in the the login window, as I mentioned in my first comment. Please try that. You don't have to reboot; just log out. The login items will be prevented from loading. Test.