Originally posted by torspoheh, I've done that a number of times. I would say for beginners it's the knight. For me, it's the queen, but it's usually cases where there is a revealed attack leading to a double attack by the queen 1 to 2 moves later.
I would say rooks are most problematic. If I have a choice which rook goes to open file, I always end up moving the rook that doesn't belong there.