Game organizers who allow preteens to play against milsimmers and laugh as they get lit up by five guns at the FPS limit at the minimum engagement distance. If your field has a huge amount of young players, have some games specifically for the kids to let them have fun away from the more intense adults and tweak the rules if they're playing with adults to make it fair and less dangerous for the smaller ones.
I actually don't really have a problem with preteens playing airsoft, since they could grow up to be good players. But what 12-year-old is going to like airsoft when all his experience is shooting his friends in his backyard and he goes out and tries to fight a guy in a balaclava with an M249, or gets pelted by a grenade launcher at MED?
While I don't "hate" this per say, I'd also say people who bring cheap Wal-Mart springers or mini AEGs to field games. Renting a weapon at the field is likely going to be about the same cost as your clearsoft shotgun and stand something of a chance at 200 feet, and it'll be a hell of a lot more fun if you're not outmatched by literally everyone except other people with as much foresight as you. Really, if your gun has difficulty hitting something across your backyard, don't try to use it for huge field games where you're shooting guys too far away to even identify properly if they weren't aiming at you.
