I realize we're talking about PS3 at the moment, but as a notice to Xbox players...
The left joystick has a tendency to reset itself near the middle, but not at a neutral position. Some of your inputs might be coming out wrong due to this. This is particularly notorious with users who 'flick' their inputs, or let the joystick reset itself at certain times.
Basically, if you know you could do certain inputs fine, or you think you're doing them good, check your controller for the issue.