People suck. That's just a fact. The unfortunate truth is that if you're living on this planet there's always going to be someone who ruins it for everyone else.
While Disney cartoons and Pixar movies may be for kids, the people making them are grown-ups who like hiding some jokes for the other adults in the crowd.
A co-star of a play he was doing snuck a real joint into the prop table and Mark ended up smoking it during the first scene of the play's opening night. He told the story on the Graham Norton show and hilarity ensues.