@rw had that happen once, linear alg exam, "answer 9 of 11 questions," I couldn't finish 2. Walked out the second the 90-minute 1/2 way time was called (to gasps of classmates who thought I was a genius, which was ironic but kinda cool for all of a second) into blinding winter sunshine...and survived. Actually felt okay, like the worst college experience I could imagine had happened and I was still breathing. If it does go that way for you, just remember to keep breathing :D