I'd also like to throw this out there on the off chance it might help someone. It applies to old apps you might have that make use of services or agents (i.e. non-UI processes) that get started when you login or when the system starts up.
OS X was making use of a technology called "StartupItems". Let's say an application needed to start a process up when the system booted. It would place a plist file (plus the process itself) in /Library/StartupItems (or in /System/Library/StartupItems, or in ~/Library/StartupItems). When the system booted (or you logged in), the OS would look in that directory for any plist files, and start up the process referenced by the plist file.
Apple has deprecated that technology for the last several years, in favor of a technology that uses a process called launchd. Launchd is a more modern way of managing those types of processes, and they use files in different folders (e.g. /Library/LaunchAgents). When I say "deprecated" I mean that Apple was giving fair notice that the use of StartupItems wouldn't last forever.
Well, when Yosemite came out the hammer fell, and the use of StartupItems was turned off completely. That means that any app that made use of StartupItems wouldn't work correctly. Hopefully, the developer would have corrected the issue long ago and started to use launchd, but you never know. I had one case of that issue occurring, and the developer came out with a fix soon after Yosemite was released (had to do with licensing).
Anyway, I thought I'd mention it. If you look in those StartupItems folders and see files in there, they're not being run as they should (of course, it could just be old crap that's been sitting around for a long time).