Yes, marking your answer sheet incorrectly, it's easy to do. Leave a question to come back to, but forget to leave the space empty on the answer sheet=good way to turn a 90+ % result into a 63% failure! Trust me, I know this well!
And in general asking for a re-mark will not be fruitful, unless you have incorrectly coded something at the top or something similar. The only grounds for appeal are against the way the exam was conducted. Usually any appeal about the actual mark will not be entertained.