Anything from making a mistake on an experiment that would ruin some scientist on earth's experiment - career, potentially - to doing something wrong with the satellite that a country was depending on for its communications, to making some mistake that could actually cost you and the crew either a mission or your lives. So there is a lot of pressure that's put on every astronaut to just make sure that he or she understands exactly what to do, exactly when to do it, and is trained and prepared to carry it out.