As I said on another thread with a correction I forgot about (it was the second worst by far):

1) Asthma attack
2) Woke up in the middle of a colonoscopy (I was under an amnesiac/twilight type drug- which obviously didn't work very well- when they took tissue sample from my colon and the pain awakened me. It hurt so bad I hit the tech and we got into a screaming match. It happened twice.
3) Abcess tooth