Sometimes trying to be helpful and tidying up your table is completely unnecessary and can even complicate things: "Putting used napkins in their water cups. I hated when people did that because even though they drank most of the water there was always a little bit left, enough for the napkin to stick to the bottom and make it annoying to clean." Stacking glasses and plates is also a no-no, as some restaurants have a no-stacking policy to prevent accidents or simply don't want to pile up dirty plates on top of each other (which then dirties the bottoms of the plates).