Things I’ve loved over the years –
Complaining that there’s no singlespeed/grand vets/fatbike/cyclocross bike/etc category. Turn up with a stupid bike by all means but don;t expect your own race-within-a-race just because you’re shit/really old and you’ve brought a ready-made excuse for being shit.
Or just complaining that cyclocross bikes aren’t allowed in your mountain bike race.
The guy who sends about a million emails demanding numerous minor changes to his and his team-mates’ entry details, just because you’re a team of apparently illiterate nitwits.
Screaming in the organiser’s face that the race “MUST BE STOPPED!!” because several people have crashed at a super-slippery bombhole survival-is-a-lottery section of the course (thanks a million, screaming-in-my-face woman, Hit the North 1, 2008)