I've never had it happen in a session (although I did get choked out by a Domme at a photo shoot after I said something smartass... but, that was more being knocked out than passing out). I have, however, come close multiple times and had to call "Red" to prevent it (in all these cases, there was barely any time to say "Yellow" to warn the Domme that I was heading towards trouble).

Basically, the symptoms for me are feeling extremely queasy and having "flop sweat" pouring out of just about every pore (I also "turn white as a ghost"). One time the cause was eating just before we started playing (the Domme was running late and hadn't eaten, so we took about 15 minutes and shared a decent size meal in the dungeon before diving right into play afterwards), the rest were because of activities that brought on very large endorphin releases (being ball kicked very hard five times in a row; hard corporal without any real warmup; and rope ties on my balls that squeezed my testes as they tried to migrate and escape). Also when my breath is taken away and I feel a large and sudden rush of blood to my crotch, like what happened when I saw Mistress Ayn for the first time (I will never forget how sexy she was in that red dress and heels), makes me feel very faint.

Because of these incidents, I actually don't eat anything within the last hour or two before a session and make sure I communicate my need for warmup before a Domme does anything truly painful to me as part of my pre-session communication. I also look at Mistress Ayn's pictures a lot, just so that I can be prepared for the sudden onslaught of Her beauty and charm when I am in Her presence.