My worst college exam was a statistics course I took. We were allowed to use calculators, so no problem, right? The problem came when the solar-powered calculator I had been using - and which worked perfectly fine in the classroom in which all of the classes had been - didn't get enough power from the diffused lighting in the large auditorium in which the final exam was being held to actually work. So I had to take that exam without the benefit of my calculator, frantically doing all of my calculations by hand on the backs of the exam papers. It took so much longer that way I ran out of time before finishing all of the questions....
Needless to say, my score on that exam was not as high as it could have been, and I went out and purchased a battery-powered calculator later the same day. Lesson learned, the hard way.
Johnathan