When I was sitting GCSEs, in my Geography exam I walked out super confident, only to immediately realise I missed out 2 whole pages of questions by accident. I was absolutely certain that it was the end and that I had completely failed Geography and all that...
BUT, come results day: I still got an A* in the paper where I missed out 2 whole pages of questions! :O :O
So even if you think you have messed up, don't stress too much! You never know what could happen! :O