Biggest mistake on the job? Hmm.....probably taking it! Hehehe
Seriously, it's probably letting my boss do what he wants on the testing computer. He changed the fonts when we were working one day. I tried to change them back, and he said, "You are going to go blind if you keep them that small! I can't work like this." I said, "ooookkk." The next day, the other doctor came in and realized that the two subjects that were tested that day....yeah, their data was ruined. Six hours worth of work that cannot be brought back.