The slip-up is right after Gohan finally defeated Cell. Gohan lost a lot of energy and is injured so he fell on the ground. Everyone (except Vegita) goes to Gohan in joy because Cell is dead. If you remember Vegita was hovering in the air the whole time, from when he blasted at Cell, to when everyone left, but when everyone's by Gohan and Vegita is supposed to be in the air, Gohan looks up to see everyone and Vegitas on the ground, but he's supposed to be in the air!