when adam is taking the tape recorder from the corpse. when he takes the recorder, you see the finger left in the air. adead body would just fall. this is pointed out in the commentary on the regular saw dvd (not the special edition) the director said that his friend pointed it out and that he 'slipped up'
Is it truly a slip-up, though, or the director forgetting an intentional move? Remember at the end of SAW, it's revealed that the 'corpse' (Tobin Bell) is alive...
Actually depending on the stage of the body it would stay right where it is. If the body was in rigor the finger wouldn't move at all. This is NOT a slip up.