Fascination over and concentration on one thing, to the near-exclusion of all other things I should have been looking at/ thinking about.
Off Florida around Jupiter, descended on the deeper side of a ledge, assumed it was about 100' to the bottom on the seaward side of the ledge close to it, as I watched bottom gradually get closer. The bottom was fascinating, for unknown reasons. While fixated on it, I didn't look at my depth gauge (who cares, I'll just level off at about 90' anyway by looking at the fascinating bottom until I'm 10 feet from it), nor had I looked to see my buddy. Don't need to inflate yet, all's well. The world was kind of serene and I had a sense of overall well-being--- until I got nudged from who knows where by the DM, who checked my air (I had plenty) and motioned me to ascend. Why?, wondered my happy brain. My buddy appeared too. Cool, I have company..
Uh. wait. What's that my depth gauge says? I'm at 130'. Why? Must have drifted farther east from the reef into deeper water than I intended. So I ascended about 30 feet with buddy, and felt more logical. Finished the dive without further incident.
I've been a little deeper without feeling narced*. But this time, why I do believe I was ;-)
*different dive, different occasion, same general reef area, same very good buddy. She pointed to a diver who must descended earlier than us, standing upright on the 150' bottom with a fixed stare, not moving. She descended from the 120' or so where we had been, and went to look in his mask and ask him what's up. I followed and was just above them at 145' in case needed. She took him (us) up about 25', and he found logic. We all continued the dive. I didn't feel narced on this one. So it ain't just depth, it would seem. Having a "job" along with someone else kept me more logical...