Johnny Cakes - S6-E8
Other mistake: AJ and friends enter the club in the city at night but the check for the liquor has 2:34 PM printed on it. (00:20:10)
Johnny Cakes - S6-E8
Continuity mistake: When Tony and Julianna are sitting down to sign the contract, he picks up a pen that is lying on top of the contract. Immediately there's a cut to the contract with the pen still lying on top of it. (00:48:00)
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.