Continuity mistake: The amount of bites out of the slice of pizza Chris is eating changes between shots.
Continuity mistake: When Chris is talking to Jon Favreau in his hotel room, Chris leans across Jon and grabs a pad from the end table next to him. In the next shot the pad is on the table again, then disappears.
Visible crew/equipment: After Tony enters the garage and finds AJ smoking pot with his friends, when Carmela walks in and comments about the smell, the reflection of the boom pole/mic moves across the car's windshield.
Visible crew/equipment: While Sal and AJ are at the batting cage, Matt starts telling AJ about the German philosopher Nietzsche. As the camera moves backwards in this dolly shot, we can see two metal T-marks on the ground, as well as the dolly track at the left 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.