I know only too well the feeling. It's an absolute b****r when you think you are doing so well then realise you have dropped an almighty clanger. This happened to me 15 years ago when I was sitting a paper for a law degree. The questions were tailor-made for me and I had completed three answers out of four in under half the time allowed which allowed me more than ample time to answer the fourth and most time-consuming question. I had already written two pages and was moving on to the next part of my answer, for which I had to refer back to the text of the scenario in the question. It was then that I realised that i had overlooked a vital sentence and had been writing for an hour about the wrong section of the relevant Act. I had to strike out my answer and start again with thirty minutes to go instead of the ninety I had allowed myself. I just made it and got good marks, but it reinforced the old instruction - Read the question thoroughly!