These sorts of issues can often creep up during updates. This one in particular happened to me as well.
As far as I can tell this is not an issue with the update itself but a result of the update interacting with a pre-existing file system condition on your computer. This can either be permissions inconsistency OR file system errors.
My steps to resolve:
1) make sure you have a current backup. I use time machine.
2) insert your snow leopard CD.
3) open System Preferences -> Startup Disk. Select the CD to boot from and click restart.
4) Once booted from the CD, choose your language. The menu bar will appear. Goto Utilities Menu -> Disk Utility.
5) Choose your internal hard drive that you normally boot from, Click "Repair Disk" This will run a file system check and repair any errors that it finds.
(Pay attention to anything in Red that appears in the log) This will either report that no errors were found, some were found and fixed, or some were found and could not be fixed. If there were unfixable errors, consult a professional immediately.
6) When disk repair is done, Then select Repair Disk Permissions.
After this is complete, quit Disk Utility, Open Startup Disk from the Utilities menu, and choose your normal boot drive. Click restart
7) for good measure, I use Onyx.
http://www.titanium.free.fr/pgs2/english/download.html
8) Install the Snow Leopard version and run it.
First it will check the disk SMART Status. Then it will examine your drive for errors. None should be found at this point. Give it your password.
9) Goto the Cleaning Tab. Make sure that at least everything in the System, and User tabs are selected. Everything else you can leave at their defaults. You may want to look over them all to see what you might be able to clean out.
10) Click execute and when it finishes, restart.
11) Download and install the 10.6.2 combo update from Apple:
http://www.apple.com/downloads/macosx/apple/macosx_updates/macosxv1062updatecomb o.html
12) Enjoy a freshly fixed Flash Experience.
