Continuity mistake: When Meadow is looking mournfully at the Scrabble board, she sees that Jackie laid "the" using the letter e from "oblique" that she laid. However, when they were playing the game, the only word after oblique that Jackie laid before leaving was "ass".
Continuity mistake: Christopher is trying to start a fire with sticks and has a cigarette in his mouth. He starts to go back into the van with the cigarette in his mouth but in the next shot the cigarette is gone and he is rubbing his hands together.
Answer: It's a mafia thing. Taking food off another made man's plate in public view like that is comparable to the guy in Casino taking off his boots and putting his smelly feet up on the table.
It is not a "mafia" thing, it's basic manners and plain gross for another person to stick their fingers into a plate of your own food. Same with the feet on the gaming table scene in Casino. It's not a "mafia" thing, it's just rude and not something a person does in public.