Twice: once when I was on holiday a few years ago, and once a few weeks ago.
A few weeks ago, I was queuing for the bus and it was late by like 20 minutes. I start feeling a bit dizzy and really hot, then next thing I know I'm laying on the floor and thinking to myself "I can't remember getting home..." Open my eyes and I'm still at the bloody bus station! Most annoying thing was I was front of the queue, and in the time I had fainted, the bus had come and everyone had walked past me to push in front! Everyone except ONE kind lady who made sure I was fine.
I had to stand at the back of the queue to get on the bus, and I just kept wishing everyone would hurry up because I felt I was going to faint again, but didn't want to sit down and miss the bus!