One of mine is when people don't even tack up their own horse... At my previous barn there was a lady who only showed up after her horse was tacked and warmed up(!!!). She'd hop on, do the jumping course or whatever, then jump off and leave, giving some one else the job of cooling down and untacking. She didn't even give the horse a pat or anything.
Another one is when people don't warm up; they just get on and go cantering off, and proceed to canter for an hour then jump 1.20 oxers, I experienced that once too.
And people who don't look where they are driving at show grounds... At a show I was with my instructors right at the edge of the ring
. I was about 6 meters away from the road. Anyway, I was mounting up to go warm up, when a car just began to back up off the road, aiming right for me and my horse! He heard everyone telling him to stop, so he did, but then proceeds to go on a rant about how it was MY fault for "standing so close to the road". I wasn't anywhere near it! And a horse in bright green is pretty hard to miss don't you think?
I can think of a LOT more, but then I'd be typing all day lol.