I've had a couple of times when I've stood up too quickly, usually first thing in the morning, after being susprised by my alarm clock. Most recently I managed to hit my head on the way down as well, right on the corner of the bed. That hurt.
The other time was playing my french horn. Massive high note, written to be played as loud as possible, for several bars. My concert shirt was a little too tight, and as I played the note, I felt little black spots appear in my vision. After ending the note, I had 50-odd bars rest, and I remember being rather surprised that I had only counted about 15, when we reached the next figure.... madness.