TIFU by not having the water actually OFF and changing the cathode on water heater.
So yesterday I decided to do the water heater maintenance that hasn't been done in the 7 years I've been here, and probably ever: replacing the water heater sacrificial cathode rod.
Google it, if you haven't done it, have a water heater older than 6 years, you should do it too, it's 3 minutes of work unless you mess up like I did.
I start with the water shutoff right above the tank, it doesn't move much, but feels solid and like it's moving easily, so I figure it's like some 1/8 turn valve and stupidly don't turn it off elsewhere. There's my mistake.
I run a faucet to relieve the lines, drain a bucket out the bottom, good so far, then take an impact wrench and get turning the rod on the top. When it gets loose it SHOOTS out along with a fountain of 200degree water (yes I keep my tank hot). An absolute gusher as I have very high city water pressure.
I ran around and got the rod, shoved it into the almost boiling geyser and managed to get that back stoppered within a few minutes, wasn't easy, had to find the impact driver I'd thrown, socket and bravery to get in that hot water in the first place lol.
Luckily I was wearing glasses, and turned my face right away, but my arms got beet red and stung for a day. Spent hours re-boxing stuff in cardboard boxes, moving everything out and mopping and drying it all out, still have fans going to finish that part off.
Moral of the story: MAKE SURE IT'S OFF. Power, water, gas... Just make sure it's OFF, way easier than the alternative. I've been duly reminded :)
TL;DR: I didn't actually turn the water off and pulled the cathode rod out causing a 200degree geyser of pressurized water.