This is in the scene where Harry (with the goblin gadget) attacks Peter from his moped (after telling aunt May he is going to propose to M.J). Peter successfully injures Harry and he is on the floor unconscious. Peter then tries to "revive" Harry. Keep your eye on the cement floor..Peters foot will shift and you can see that the cement is actually a patented carpet. "the cement" gets stuck to Peters foot and folds up.