I'm missing an entire evening of memories from earlier this year. Turns out I completely lost my shit over something (I can't remember what, and nobody seemed to want to tell me), and I wound up taking a term off school as a result. Not sure if that was a loss of consciousness or a loss of memory.
Medical occasions: Blood tests always make me black out, no matter what. It's partially a physical aversion to needles (not a fear - I'm fine with the idea, but it just happens) although probably more to do with the blood loss. Vaccinations do this occasionally, but that hasn't happened for a while. Anesthesia's another obvious one, although I've only been put under for jaw surgery and a groin operation (

).
More recently: Two weeks ago I dropped MDMA whilst already under the influence of psilocybin mushrooms. After maybe twenty minutes of giddiness and fun side-effects, I went to lie down and woke up twelve hours later with the worst headache of my life and an odd detached sensation in my limbs. My friends told me I wouldn't wake up when slapped and they were convinced I was dead. I remember being convinced that I was going to kick it that night, too. Not going to do that again.
Jam it back in, in the dark.