I noticed this weird problem while playing Black Ops on my PlayStation 3.
The problem happened when attempting to sprint (L3 button), usually at a time when I was about to shoot somebody. Instead of sprinting in for the kill, my controller would inexplicably switch weapons (normally the triangle button) or whip out my claymore (d-pad up button). Which then would usually result in me being delayed and getting shot in the face. This would happen about a third of the time. Not awesome.
This didn't happen in other games, and it happened with both controllers, so my first thought was a software bug.
However, I finally broke down and bought a new controller. Problem solved!
Evidently, the L3 button wears out on PS3 controllers in such a way that the signals are occasionally interpreted as other buttons. For whatever reason this problem is more noticeable with Black Ops than other games.