Unfortunately, my latest stupidity was a lot more severe, but maybe it'll make you feel better.

I have 2 different math courses at uni, called Math 1 and 2, for the last exams I signed up to write math 2 and learned all the things I thought I had learned for the previous try, which I had failed. I was quite confident on the day of the exam and happy that I'd most likely be done with one of those two math exams until I saw my friends and they talked about completely different stuff... Needless to say I was out of the room very quickly since I had learned the wrong material.

I don't know why but my memory was off by half a year(the two courses rotate like that), a simple look at the webpage or so would have opened my eyes before it was too late but for some reason I trusted my memory to be correct and didn't check.

Shouldn't happen again.