Dragonball Z - DBZ Return of Cooler
During the 6th DBZ Movie, when Goku and Vegeta are in the Big Gete Star with Cooler. This is where I saw the slip-up. Goku is talking to Cooler's head, (he is standing and Vegeta *non-super saiyan* is on the floor unconscious) Goku then transfroms into a Super Saiyan, and starts charging a blast. While he is charging this we can see Vegeta on the floor. He is still unconscious yet we see that he is a super-saiyan, impossible, seeing as he was knocked out as a regular saiyan, and suddenly him being super saiyan lying on the floor.