If you’re a Mac user of the FinderPop utility, you might notice that Photoshop CS5 can crash when you choose Select > Inverse or other various menu items. If that’s the case, try downloading the FinderPop 220.127.116.11 Beta as it contains a fix for the problem.
It’s also possible that you’re seeing some weirdness if running OS X 10.6.5 or 10.6.6 (e.g. seeing a “Delete the Adobe Photoshop Settings file?,” tools behaving as if the Shift or Option keys are pressed). Something got broken in the 10.6.5 update, and the Photoshop team is working with Apple on a fix.