I answered the wrong question in my RS GCSE exam (you could choose between 2 questions and I answered parts a, b and c of one question and d of the other) so that cost me a good 8 marks or so... I only realised when I was checking that I'd ticked the right boxes in the last two minutes of the exam, so it was far too late to fix my mistake!
I also put 89+1=100 near the end of a show-that question and couldn't work out why I wasn't getting the right answer... Only figured that one out when talking to my friends after the exam!