I can't STAND it when horses move off when you're mounting. It's seriously my most major pet peeve, because I think it's both dangerous and preventable...even the 'hottest' horse can be taught to stand until you're settled in the saddle with your feet in the stirrups! I also can't stand people who let their horses trot away from the mounting block. THat's just asking for a major injury, as far as I'm concerned.