I tried to break down a Solana with a broken back pane, figuring I could get a new piece of glass cut and re-assemble it. Apparently using a mostly dull paring knife the try and separate the panels was a bad idea, the knife slipped out of the joint between the panels, hit my thumb, cut it to the bone. That was 6 years ago, I now forever have a scar from it. Of course when it broke I happened to come home just in time to see saltwater dripping directly into my surge protector, lots of sparking and sizzling. I now keep my strips of plugs attached up on the wall, or inside of my stand on a shelf where no water can hit it.
Had a short in a return pump that was electrocuting my tank. I found out the hard way when I reached into my tank. I can't imagine what the poor fish and corals had been going through with that constant current running through the tank.
And of course bristle worm stings, I'm apparently very sensitive to them. A couple times my fingers got so swollen that the tip started to turn purple and I could feel my pulse in it, like having string tied tightly around them. Each time that's happened, the day I decided "If its like this tomorrow, I'm going to the doctor" is when it starts to recede. Yet I still keep grabbing rocks without gloves on. I should really just invest in some of those thick elbow-length gloves.