Continuity mistake: After Gru tells the first minion "Minions assemble", that minion slides down a pole and relays the instructions to other minions. After he does this you can see a minion standing on the right of a blue tube and he jumps in, after the camera begins to pan to the left, that same minion instantly appears back at the entrance of the tube. Probably an error in the animation sequence which returned the minion to his starting position when his animation was finished.
Rockmonix
11th Jan 2011