I have done this multiple times throughout high school, normally at the moment when it costs me the most, I still have nightmares about it......
2+2=6
Three years in a row I did that on a maths exam, the first time it was on the last line after a whole page (and I mean a whole page filled) of maths and it was the only thing that I made a mistake on. Everytime it happens now I try to forget it, but it still comes back to haunt me.
Probably not the most idiotic thing I've done but I KEEP MAKING THE SAME MISTAKE AGAIN! 
HAHA! I guess everyone has those stupid math mistakes that you never get over.. for me: 
Hey, at least your teacher didn't decide it would be a good idea to show the class...
In year 11 Physics I was doing a question and at the end I had to do something like 60 + 40 = 100
I then realised that the 40 wasn't supposed to be there (miscalculated previously) and changed it to 90 (or w/e it was)
Unlucky for me I didn't change the answer so I had 60 + 90 = 100
As my teacher was marking my test at the front of the class he burst out in laughter, looked at me and held up the paper and said something like "are you serious!?".
He then proceeded to tell all the other students in the class to come look at the paper and they gathered round to check it out.
Totally not cool... lucky for me I'm a pretty easy-going guy and got along well with everyone in the class, but still... what kind of a teacher does that?

#scarredforlife