On my first trip to Cozumel I did a really dumb thing. Well, more than one.
On my very first back roll entry from a small boat, I over-rolled and I came up directly below the boat, smacking the top of my head on the ridge running along the keel. That will get you seeing stars.
Not to be outdone by myself, I them hurt myself again getting back into the boat. Once, climbing back into a 8-pak boat on a folding ladder, I held onto the ladder, but not onto the post for the canopy. When I got to the top of the ladder, it pivoted forward, dumping me on my face on the bottom of the boat. I landed square on the face of my mask. My shins were at the ladder, with my feet in the air. My tank was smashing the back of my head, holding it at the bottom of the boat. With my feet in thin air, and all that weight on the back of my head, I had a heck of a time getting out of that predicament.
Also, when I struck the deck, my little knife popped out from the back side of my console (Aeris). I got a little bit of a "look" from the guide.
Now if you REALLY want to know about stupid, you need to read about "Ed the Diver".
Wristshot