Mine is people who buy horses because "they're a good deal" and think that all horses are the same.
It just grinds my gears. There're people by me like this. They up and bought four horses without any research. One horse they bought is barn shy, only been saddled once, never been ridden, and they bought it for their 7 year old grand daughter.
One had cancer and died on them within two weeks of them buying it.
A third needs a highly experienced rider.
And the fourth is just disrespectful and needs a firm hand. Which they don't have.
Just irks me to no end.