I'll share a few of mine
When people think that changing a bit will have magic effects on thier horse.
When people try to ride a horse in a rope halter that they can barely ride in a snaffle because they have a "connection" with thier horsey.
Teenage riders with a year of experience that try to start colts and lecture about the "right" way to do things.
I really hate when people leave halters on thier horses when turned out in a pasture. Not only is it unsafe but it's trashy looking too.
Nothing I have listed is meant to offend. Please share some of your peeves.