It's only happened a few times. Once was in New Orleans on Mardis Gras when I was already extremely shitfaced, and then took a bunch of shots in quick succession. The next few hours are blank. Fortunately I had a friend watching out for me.

Aside from that, my tolerance level for blacking out seems to be very high. The threshold lies beyond the puking, stumbling, overly emotional stages of failed moderation.