Only once. I got my measles and hepatitis B shots one after another. They say combined it can usually cause to being light headed. Well, as I walked back after signing some papers to get my lollipop, my vision started closing in on me. At the time I had no idea what this meant, but I just... blacked out. I fell forward, and hit my brothers shoulder... he thought I was joking and let me fall. Then I fell face first, and remember waking up with everyone around me as I lay on the tiling. Then I don't remember walking off or getting in the car, or the ride home or anything... I just remember waking up on the floor, then my memory blanked, and I woke up in my recliner at home. I busted my lip and got a black eye... but it was no ordinary black eye. It was just on my upper eyelid, so it looked like I was wearing mascara (i think that's the eye makeup shit? I have no idea, this one girl used to tease me saying it looked like I was wearing makeup).
That sucked... now when I start 'tunnel vision' I understand that I need to sit the fuck down before that happens again. We learn from our mistakes :)
I was also terrified of getting shots for some period after that >_<