Continuity mistake: Meadow takes the carton of milk out of the fridge and leaves it on the counter. Tony gets up and opens the fridge and the carton of milk is back inside. (00:11:01 - 00:11:53)
Continuity mistake: In the scene in the pizza place, the coffee pot's handle moves around between shots, and the design changes completely. Sometimes it's round then triangular.
Continuity mistake: When Silvio Dante is talking to Richie Aprile, from a back shot Silvio rests his head to the back but in the next shot it's straight and down.
Continuity mistake: When Janice goes to Livia's mailbox and retrieves the envelope with her disability check, the words "TOTAL DISABILITY" only appear on the check in the close-up, but not in the previous shot. Additionally, the silver screen door is gone between the first wide shot when Janice pulls the mail out of the mailbox, and the next close-up as Janice looks through the mail in her hands (she did not open the screen door between shots).
Visible crew/equipment: After Carmela and Janice apologise to each other, Carmela asks Janice to lower her music, and when Janice closes the bedroom door, lighting equipment is visible at the right side of the screen.
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.