He also spits on Elmo! Clearly he's joking around, but this is pretty funny in a very twisted way.

I'm not sure what year this was, but it's safe to say it this was from a long time ago. It's behind the scenes from Sesame Street when Pesci had enough of Elmo's babbling and spits in his face. It's hard to tell if he actually spits, but the look on Maria's face is not that of happiness!

Further on in the clip you can see Pesci hit Elmo in the face with blueprints, and jokingly requests that they take the kids off the set while they film him.