I've had a near blackout and a samba, both while doing wet statics. I believe that purging a little too much might be the cause.
The first time it happened was when I started to include purging before statics, I didn't realize that by overdoing it I could really eliminate much of the urge to breath. Right at the beginning of the hold I experienced some of the tell tale signs of having low CO2. My fingers were very tingly, up to my knuckles and I was a little dizzy. At that point I had never experienced a blackout so I figured I would just go ahead with the hold and I would know when to come up.
Needless to say that this static felt really comfortable, I only started to feel the contractions past 4 minutes (at that point I usually got them near 2:30) even though my buddy said I had fluttering contractions or something like that that started around 2 to 3 minutes. The contractions were there but they were not uncomfortable, it wasn't like I was fighting them. All I remember of the end of the static is that I was listening to some people taking around the pool, I was able to signal to my buddy I was ok. Eventually it felt like the contractions had left completely and I could just stay in that state, without breathing forever. It was like falling asleep. At that point I was kind of out of it but still able to respond to what my buddy was saying. He asked for a signal, I gave it to him. He told me to put my hands on the edge of the pool, I did it. He told me to come up, I did it. The last thing I remember hearing is 'come up' .
I was told that I took my head out of the water but wasn't breathing, my buddy just took off my mask and only then did I start breathing. After a few breaths I was alright but had no idea I had just had a near black. I just thought I did a great static and it felt good.
My second experience was during the PFI clinic and resulted in a samba. My purging was much less aggressive so the start of the static felt much better, I was not dizzy but my finger tips were a little tingly, as they usually are. It felt like a usual static for me, I think the contractions started around 3:30. I remember having to really fight the contractions beyond the 5 minute mark. I decided to keep my eyes open this time in the hope of getting any signal to come up before its too late. I did, my vision was a bit greyish and blurry, kind of like when your looking out from a speeding car. I thought to myself "I should really come up now" but I wanted to wait another 15 seconds to reach 6 minutes. The contractions were getting very strong and bubbles blew out of my mask twice, then I was told to grab the edge of the pool and come up. This time I remember everything pretty clearly and I had a pretty bad shake. I kind of fell back once I took my head out of the water.
Given that I've only had two near balckout/lmc experiences and they were quite different I haven't figured out any specific patterns that lead to a blackout for me. For this reason I agree with Tylerz statement,
I believe that, unfortunately, if a person has not had a blackout/LMC then unless extremely experienced with numerous attempts at pushing their limits, they are unlikely to have solid grounds to comprehend for their body when they are close to blackout.
Even after reading about signals that lead to blackout here on DB, I had no idea it was going to happen to me when it did. Even if you play it safe and come up at the first urge to breath you could still be susceptible to blackout. You may never feel the urge. When I had a near blackout, I would have just stayed there motionless in a very relaxed, comfortable state if it were not for my buddy who told me to come up.