So I picked up uncharted 2 yesterday and played through the first chapter and noticed I got signed out of PSN. I got paranoid so I stopped and saved and backed out just to be sure I didnt loose a save file. I signed into PSN again and about 10 minutes later I got signed out again. 
 
Has anyone experenced this? It gave me an error message but I didnt write it down. I've never had this happen to be on any other game either.

playing offline won't hurt anything.  I have that happen sometimes just because my internet skipped a beat or something and it signs out.

I suggest you retry doing whatever it is you did that presented an error message, write it down and google it. I did notice sometime last night I kept being signed out of PSN but a simple restart of my console fixed it.
 
Chances are restarting the console and writing down any error code that appears will help you pinpoint the problem and fix it :)

I dont think it hurts anything,but i was buying a booster and i got signed off.....when i sign back on and into mutli player...it said my game file had been modified and single player wouldnt carry over....i have like $350,000 in single player and only like $15,000 in multi...but now all that shows up is the $15,000..I've only bought 3 boosters..i was like wtf!