I absolutely hate poor tale manners.
Napkins belong on your lap, or on your chair if you get up. They do not belong on the table, especially after you have used them to wipe your mouth or nose.
And I get really grossed out by people who scrape the dishes at the table so that there is a stack of plates with a sizable pile of chicken bones on top. The only difference between the chicken bones on the plate and those in the garbage can is location. Garbage does not belong on the table.
And serving pieces are provided so that you don’t have to use your own fork or your hands to take food off a platter or out of a bowl.
This one is slightly off topic since it’s not a politeness thing, but it does frequently happen at the table. I want to scream every time somebody says, “This ____ is yum.” “Yum” is an interjection. To use it properly, take some food, swallow it, then say, “Yum.” That’s it. Otherwise, if you really want an adjective, the word you are looking for is “yummy.” It is correct to say, “This ______ is yummy.”