1. When people smack their horse as a praise, you know, like "good boy", *smack smack smack*, Instead of actually thinking about the fact that a horse doesn't really see smacking as a praise... I dunno, maybe that's just me, but a scratch on the withers is what a wild horse (or a domestic horse for that matter) would do to their horse friend, so why wouldn't we give them a scratch on the withers for praise?? You should. You should.
2. People who yell at their horses.... yeah you're horse really understands what you're saying when you yell it even louder, and ESPECIALLY when you scream loud enough for the neighbors 600 acres away to hear. I'm sure the horse is thinking "Oh, I get it NOW! Thank god she yelled, I couldn't hear her! Wow, I'm so glad I understand!"
Those are my two biggest, but I have MANY more... and I want to see what other people think!