I'm guessing this thread is based in part on a post I made about a diver who could not remember an entire dive because of a bout of transient global amnesia (TGA). TGA is extremely rare to begin with, so it would be extremely unlikely to happen on a dive. It is possible the incident to which I referred is the only one in history, although it is also possible that people have had smaller attacks during a dive without realizing it. (They leave no trace.)
In terms of narcosis, I don't think it is so much not remembering dives or significant portions of dives; what I have seen mentioned is forgetting details of dives. The one case I experienced was when I was doing a fairly long decompression dive at about 130 feet while on air. After close to a half hour at that depth, I noticed a very slight free flow in the regulator in my mouth. I carefully adjusted it to get the best flow without the free flow. I congratulated myself on being very aware and very clear-headed about it, even though I was in a range where narcosis could be an issue. I was certainly not narced, I thought. At that point my buddy flashed me with his light to get my attention. He wanted to know why I was breathing from my alternate instead of my primary. I had no idea I was. At some point during the dive I had decided to make the switch, and I did not know when I had done it or why I had done it. I suspect it was because I had had some issues with that regulator's tuning earlier and wanted to check it out, but I didn't remember doing it.
