One of the more serious ones I've had was before an exam in January 2013. It was for early entrance to the PhD programme in Cambridge, and shouldn't have been too much of a worry as I was well aware that the competition was probably too tough for me. But as the day of the exam came I basically became as anxious as I've ever felt. Could barely eat a bite of food, and was on the verge of feeling physically sick.
The usual rule is that when I'm actually doing the thing I'm nervous about I settle into it, and it was the case here. In the end did about as well as expected (which was not well enough), but felt OK during the exam at least. For whatever reason, I seemed to get more nervous with exams as it went on rather than less despite all the experience I've had with them (and the ones I did in Summer 2013 were pressure-free as I'd already got into Edinburgh unconditionally, still felt rather nervous about them though).
I spend a lot of the time feeling anxious anyway but that's "just" general stress. The day of that exam was pretty much a full-on anxiety attack.