The scene in Pulp Fiction where Jules and Vincent turn up at that flat to collect the suitcase, there's a moment where a guy just appears from nowhere and unloads a clip and all the bullets miss them both. On the second or third time I watched it I noticed the bullet holes were already in the wall in the frame just before he starts shooting.

I also remember realising (on probably my millionth watch) of Rocky IV that there is a very short clip of Apollo in the ring as he's waiting to fight Drago where he points at him and his boxing gloves have miraculously disappeared.

I class both examples as some of my finest achievements in life.