With regard to the Buffet. I hate it when people are really slow deciding what to get, and then when you pass them, they give you an angry look as if you are cutting infront of them in line.
Part of the problem is poor buffet design, but I know what you mean. On many ships they put the toast at the end of the main breakfast buffet lines. So if I just want another piece of toast and nothing else I have to get the look of evil from those back in the line getting their scrambled eggs and bacon. It's like "you are supposed to fall in line and wait 20 minutes until you get to the toast!"
For the serving islands they have for some buffets, there is a misconception that these should be lines. To me they are more a "go to the tray you want and help yourself" set up, but the "line up" mentality seems to rule.
So the pet peeve for this is: poorly designed buffet lines