Continuity mistake: After Voldemort's soul flies through Harry, he crashes down on the stairs and his hand sticks out over one of the steps. In the next shot of Harry lying there, his hand no longer lies over the step. Harry is knocked out and could not have moved. (02:13:20)
Continuity mistake: After the final confrontation Harry has scratches on his face, particularly one on his chin. When visited by Prof Dumbledore the chin scratch is barely there but in the scene in the refectory it is back and appears larger than before. (02:14:00 - 02:17:50)
Continuity mistake: As the students board the Hogwarts Express to go home, the cuts on Harry's face, particularly near his eye, differ in severity during this scene depending on the camera angle in a given shot. (02:21:55)
Continuity mistake: While Harry and Hagrid stand beside Hogwarts Express, in the shot looking up at Hagrid as he takes the album out of his pocket, his thumb is on the back cover as he begins to say, "This is for you," yet in the next shot of Harry taking the album, Hagrid's thumb is on the front cover. (02:22:05)
Continuity mistake: Daniel Radcliffe, the boy who plays Harry Potter, has naturally blue eyes. In some shots Chris Columbus, the director, changed Daniel's eye color. But Chris C. didn't want to use that the whole time. It's not a lighting problem.
Answer: Petunia or Vernon would have ran to a zoo employee and told them what happened. The employee would then open the door to the snake exhibit letting Dudley out.