Having coughing fits. Happened twice now, in a Geography exam last year and in the Maths exam this year. Also, when I forget to take my hayfever tablet and have an exam in the summer my nose tends to run wildly so there I am sniffing, snorting and trying to prevent snot from leaving my nose.
The absolute worst thing so far though was in a Spanish reading exam and finishing thinking I had all the time in the world left, but when I decided to go over my paper in the last 3 minutes of the exam, I realised I'd left out the last section which has the most marks in, so I had to very quickly read the extract and answer the questions... some I had to guess because I just wouldn't have had time otherwise.