When I was younger, I would pass out from dehydration, but they were full-on, blackout, piss-my-pants fainting.

More recently (as in a few years ago), I ended up passing out from what I think might've been possible anemia due to poor diet. (I definitely wasn't eating well, and nothing like this has happened since I started paying better attention to what I'm putting in my body.) I was standing up, but I felt dizzy and my vision was starting to go black, so I sat down (since that's what I was taught to do as a kid from the aforementioned fainting spells), but I ended up blacking out anyway - fell out of the chair and smacked into tile floor head-first. Ended up with a golf-ball-sized lump on my forehead and calling out of work sick that day.

A few years before that, I passed out when giving blood for testing for anemia - which my mom thought I had when in reality, the blood test showed that my lethargy and general crappy feeling was because I had a mild case of mono. I do seem to be prone to anemia, though, but that particular time was most likely because I was still recovering from being sick.