Oh, so many. Either everyone else is bashful, or I'm just really stupid...
This is just a random selection from the hall of infamy.
Descended with snorkel in. And not just once or twice...
Giant stride without fins, having observed all the OW students doing their buddy checks.
Entry with air off, ditto.
Got halfway to dive site, after lecture to students on importance of checking they have all their kit because of the length of the boat trip, before realising that I have. Not. One. Solitary. Item. Of my own kit on the boat.
Arrived at dive site for a deco dive and remembered that backplate and harness are still on single-tank rig at shop.
Wondered why I was having such trouble holding a safety stop. When I surfaced, wife (who was DM-ing on boat that day) suggested the dive might have been easier if I'd taken my weight pockets. (One of many reasons I use a bp/w when single tank diving nowadays...)
Tied self up in fishing net trying to free a cuttlefish.
Later tied self up again in same net as part of team removing it from reef.
Had a valve jam open when I was doing a shutdown drill. Such was my determination to complete the drill, I sheared the knob clean off. Dive over.
Forgotten to reset computers for different mixes untold times.
Forgotten to tell computers I have completed gas switches several times. Don't do technical dives with computers any more.
Allowed wife to follow me into bottom level of the Fujikawa Maru's engine room after her primary light failed, without her deploying backup. Well, it'll be fine, I'm carrying a 50W HID and we're just going round this corner to the engineer's station. Turns out light doesn't travel round corners...
'Shark calling' in spots where my back isn't against rock or reef. The ocean is three-dimensional, and the bit behind you is still there even if you can't see it. Stopped doing that after an 8-foot Silvertip came over my shoulder and knocked my mask off at 50m... I don't know what haunts me more: recovering my mask and seeing what had just hit me, or the sound of the wife's laughter while I was scrabbling after my mask.
Arrived at dive site and found I'd brought empty tanks.
Tossed mask into small boat before getting back on board. Except I actually tossed mask OVER small boat and into sea on other side. Gone.
Back-rolled into three feet of water (boat had drifted slightly while I was briefing OW students and driver hadn't noticed). Landed on tank, fortunately.
And talking of boats, swept up to the jetty in one of our 23-foot skiffs, brought her alongside perfectly, was stood chatting to customers about the dive they'd had when I realised I was close to knee-deep in water. I'd taken the bung out to empty the bilge during the trip back...
There's probably more.