I was once sitting a 3 hour Law exam. After 1 1/2 hours I had completed three questions out of four. That gave me a whole 1 1/2 hours to do the last question. I was feeling rather smug and thought, "I'll knock 'em for six with this last answer". So I started writing, and writing and writing... Glancing at the paper again to check a detail I realised to my horror that I had misread the question and what I had written was totally irrelevant. I hastily scribbled it out and had to start all over again. I made it with two minutes to spare.