Back when I was taking my Biology As I noticed approx halfway through the exam that my friend had finished, he was just sat there looking bored. I continued doing my paper, finished it with about 10 mins to spare. At around 5 mins left we both decided to have a flick through and check we were happy with our answers/no stupid errors.
He suddenly starts writing frantically for the remaining 5 minutes - turns out he's accidentally turned two pages and hadn't finished a third of the exam paper, which is why he finished so early.
Idiot.