I had an appendectomy just before the final first-year Modern Physics lab and had to come back in to do a make-up lab. Sure enough it was the slightly-messed-up lab where the results should in theory look exponential but come out linear. I, naturally, drew an exponential curve through the points. Lab instructor decided to grade it right there before I left and tore a strip off me.

Very valuable lesson, although it sure did suck at the time.