Someone picking food from your plate or drinking from the glass you are drinking from
This is definitely bad manners and bad dining etiquette. Not a very common behavior but friends tend to want to do it at times especially when at a bar or something. They might be drunk or thinking that you won't mind. One of the things associated with this is bar nuts, where everyone eat from the same bowl. Buffets are also famous for people picking out food with their hands, but this is not the same, as your plate belongs to you and maybe you feel violated when someone who is not your spouse or a family member pick food from your plate or have some of your drink.
Eight out of ten said no to the plate but all ten said no to the drinking part.