People who join a forum without understanding its focus and then complain that they are ignored.
Or people who join a forum and whine loudly when the forum community doesn't always stick to topic. One I remember was for Polish genealogy. One member, perhaps a bit tongue-in-cheek, asked if pink lawn flamingos had some cultural significance to Poles. Her in-law's community, largely Polish, seemed to be Flamingo Central, because almost every house sported one or two.
And a brand-new member started screaming "I DIDN'T COME HERE TO TALK ABOUT FLAMINGOS!!!!!"
Same newbie had also complained when someone mentioned that she had just inherited a Polish cookbook, and someone else asked if it had a recipe for X.
Newbie was told that online community standards were determined by the community, and if they chose to discuss things that were peripheral to the main topic, it wasn't the place of a newcomer to complain.