Most of them are small, like not thinking to put a sheet down on a Reeves stretcher to make it easier to move a patient to the ambulance stretcher later one, or tying bandages in such a way that the un-padded side of a splinting board ends up against the patient.
But then there are the big mistakes. The deadly ones. The ones where, as a student, you sit at the end of the exercise wondering "Do things really go bad that quickly in real life?" [The answer, by the way, is unfortunately 'yes'].
As we come to our final exams, it almost seems that the instructors are purposefully trying to have us kill our patients, because ... well, let's just say that they can be sadistic at times. They are absolutely giving us every opportunity to inadvertently 'kill' our patients as we role play through scenario after scenario.
The good news, of course, is that this is all pretend and no EMT students are harmed by our errors.