I work at my barn and I'm there all the time. I could tell you loads of stories of "..wut" from boarders because I know almost everything that goes on around the barn.
This one isn't exactly a bad boarder, just..out there. One lady has some extreme paranoia issues. She needs professional help, but from what another boarder (a friend of hers) said, it comes and goes. When it's "off," she's very nice, but when it's "on," she's nuts. She thought she was being followed so she calls the trainer. I'm not sure why you would call your horse trainer if you thought you were being stalked, but she does. Trainer told her to call the cops if she felt threatened, but it was just strange. She leaves 2 minute voicemails about nothing for said trainer now.
This woman thinks her gelding is being poisoned now. She has no evidence and his health hasn't remotely changed. Oh, she also hates me. She thinks I'm abusive because I was lunging this pony of my BOs' who pulled out, tripped herself, and fell. And because I whack my horse if she deserves it. She refuses to ride anywhere I am.
As for a bad boarder, there was one guy who just left. He owns a 17.2hh WB who has zero respect half the time. He travels often for his job, so he comes out rarely. This gelding has broken gate latches, fences, beat the crap out of other horses, broken his stall door several times, cross ties, his feed bin in his stall, etc. Owner refused to pay for any of it without receipts. He's bolted on everyone working there at least a handful of times.
It took months before the owner admitted it was because he's not out often and doesn't work this gelding. He'd be a great horse if he was worked daily and had a job. He's green at best. Finally, the owner realized his problems when whenever he'd come out, he'd bolt out of crossties and go tearing around outside. Of course, he wouldn't discipline him, just a wagging finger and "Oh, you naughty boy!" This was every time the guy came out.
His girlfriend also boarded there and she could have worked him, but he wouldn't let her because she's a woman. Yep. His girlfriend has two wonderfully mannered horses, she's just crazy. She'd drive across town at 10pm just to make sure we watered her horses and creep. We have never not watered them. I don't even know. She just broods around if anyone else is around also. Occasionally I'll get a "Hi" back when I say hi. Now just ignore both of them. She's leaving within the next week also.