Yeah, it's one of those unwritten rules that doctors know, but only disclose off the record. I think it's from lack of oxygen to the brain, similar to "almost drowning," but anesthesia is powerful stuff, so who really knows in any individual case. The clearest example I have is from my friends mother who rapidly deteriorated over from very active and fully alert to obvious forgetfulness and beginning dementia signs over the next 8 months, losing her independent living as a result.