My Samsung Galaxy S was working well, but about a month ago it turned itself off, and upon restarting went through the initial setup again (language, keyboard, etc.). At the same time, lots of apps started to fail, and were force closed and relaunched by the OS (Swiftkey, Agenda Widget, and others). All my other data was intact.
Eventually, I reset the phone (losing all the texts and my Angry Birds progress :o) and reinstalled the apps and set up my sync accounts again.
The phone repeated this process three times in a week, so I requested a return, but then as I was waiting for a returns envelope, everything seemed to be fine. I decided to see if it was a glitch and all was well until this afternoon, when it turned off again (I checked and the battery was pretty full) and has started doing the application force closes again.
Running Froyo from the official Samsung update via Kies.