Only once. I was about 15 at a friend's house and he was giving me cocktails with spirits in (I never drank spirits, only beer and occasionally wine). The last thing I remember was not being drunk, then I smoked a joint and remember waking up the next morning upstairs asleep on his bedroom floor.
To this day it is the only time I've ever blacked out. I have never really understood why it happened that one time and never again.