Once again, you fail to realize that people aren't perfect, and just because they make mistakes doesn't mean they're bad. Plus episodes need conflict in order to be engaging, so she has to make those kinds of mistakes or there would be no episode.
Besides, the other mane six ponies screw up with their elements too. Rarity has been greedy and selfish many times, AJ lies all the time, Fluttershy can be a massive jerk sometimes, Pinkie has been more annoying than funny, etc. I don't see you complaining about any of that.
Only people with no sense of humor would be offended by that.