Sad to say, I have passed out, but I have learned from it and vowed to never repeat it again.
It happened last december. My wife and I were passing through an economic crisis since I was waiting for a check that came in late due to a strike at the bank. We had been eating really badly for 2 weeks and got the flu. I played 4 straight concerts in diferent cities: one solo recital, one recital with a violinist (Beethoven mostly), my piano exams and the last day, the Shostakovich first concerto with the schools orchestra. Id had rehearsals all week, with the violinist and the orchestra, not to mention my theory exams and practice time. AND I was preparing an important audition.
Anyways, I played Shostakovich even though I was running a 102 degree fever and I was really dizzy! (but I wouldnt have money for the doctor till I got payed, which Ironically was the same day this happened).
It went extremely well, even though I was in a daze most of the time. The last movement requires a HUGE physical investment, so when I got up from the piano and came on to stage for the second time (although I had no intention of playing an encore) everything went black and I fainted right there. I woke up backstage and got up only to faint once more. I woke with a paramedic above me. He told me I had the flu combined with anemia... It took me about 3 months to recover completely.