This slip-up happens in the graveyard scence towards the end. When Harry is pinned to the statue, Voldemort states that he can touch Harry, so he(Voldemort) goes up to Harry and touches him on the forehead. When they show Voldemort's face from the front, you can see on the left his bicep muscle sticking out. Then they show Harry. Then back to Voldemort, but this time Voldemort's robes are now pulled up and covering his muscle!

Magic?

Editor's Note: I've seen this slip-up submitted somewhere before.