Guy Walks Into a Psychiatrist's Office - S2-E1
Revealing mistake: Christopher is seen sniffing some lines of coke, but he is just moving the dollar bill up and down not sniffing anything at all.
Knight in White Satin Armor - S2-E12
Revealing mistake: While Christopher and Furio are dismembering Richie Aprile on a meat saw, you can see that the blade of the saw is still, but the blood is splattering all over the place, and you can hear the sound of the meat saw cutting through flesh. (00:49:55)
Knight in White Satin Armor - S2-E12
Revealing mistake: Noticeable blood pack under Richie's shirt when he's shot by Janice.
Knight in White Satin Armor - S2-E12
Revealing mistake: Richie is breathing on the floor, despite having been shot to death. (00:46:58)
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.