I work at barn with six current boarders, most of which are really nice people. We have had some pretty nasty boarders though, both at my current barn and the one I used to work at.
Of our current boarders I only have a problem with two. The first boarders own a lovely little pony that they bought for their ten year old when I decided she wanted to start riding lessons. After owning her for a year the kid is terrified of the pony (who any other little girl in the barn would kill to own), and refuses to ride her. Mind you, this is the most well behaved and safest pony I have ever seen. She fell off once. Anyway, the mother is a total control freak and we've gone through all kinds of crap with her about using the pony in lessons (which she told us we could do), and now she wants to donate the pony, but still wants to have control over what happens to her, who rides her, etc.
The second woman is really nice, and genuinely loves her horses, but is a total pain to work for. She gets really mad if her horses don't go out at the exact time she wants them too, and stay out until the exact time she wants them too, even if they're running and screaming out in the field. She can't seem to get it through her head that we have nearly 30 horses and limited pasture. Every horse needs turn out time, and she pays the same amount of board as everybody else. She has made multiple complaints that her horses' stalls were dirty all the time. I'm sorry, but when I have 16 stalls to muck and 15+ horses to move, the ones who go out after everything else is done are not my top priority. She also goes into her horses' stalls after they've gone out and pulls down any bedding that has been banked against the wall, then puts on our whiteboard that they need bedding. She often covers pee spots with the bedding she pulls down (her stalls get mucked before the horses go out so sometimes they pee or poop again). She also brings her obnoxious dog to the barn all the time.
At one point we had a boarder who was with us for just over a month. First time horse owner, in her late 30s, and thought she knew everything about horses. Her horse was a lovely little mare with wonderful manners of whom the owner was terrified. Which was odd because she would go on and on about how perfect the mare was. No hay was good enough for her horse, no water clean enough, no field big enough, no horse nice enough to have out with her. My barn generally has a really nice atmosphere, and all the students, staff, and boarders feel like family. I have never felt so much like staff than when this woman was around. After the first week she stopped acknowledging my existence unless she needed something. She would walk in, I would greet her in a friendly manner, and she would walk by without looking at me. She was always very friendly with the boarders and older students. And I know you shouldn't judge the names of peoples' horses....but hers was called 'Dollbaby', and she got mad if you even shortened it to 'Dolly'.
I saved the craziest for last :) There was a woman at my old barn who was an absolute nutcase. Her horse was every bit a stereotypical mare. Moody, mean, spooky, aggressive. The owner talked down about everyone at the barn behind their backs. She was incredibly rude to anyone else who tried to ride in the arena at the same time as her. She yelled at one of my eight year old students one day for letting their horse stand too close to the circle she was riding on. I got yelled at multiple times for bringing in or turning out horses while she was riding because 'it really distracts' her horse. You show. There are going to be distractions. No, I will not put off feeding for an hour so your horse won't turn her head to watch me walk by. She also screamed at her horse whenever the horse didn't behave perfectly. My BO ended up kicking her out after she broke another boarders jumps by throwing them out of the ring.
Yeah, there are some real crazies out there. We do have some wonderful boarders though. I love most of them, even if they don't always muck their horse's stall or help with feeding and turnout. I've actually noticed that the real crazies tend to try to do more for their horses than the nice, easygoing boarders.