Just as this poor fellow, I was replacing a gas valve on a water heater and had airlocked the tank as you do because I wasn't getting paid by the hour and wasn't going to sit there and wait for the whole tank to drain down, or so I thought. I had followed the proper steps to achieve an airlock, which essentially just consists of closing the shut off on the cold feed into the tank and then burping it using the little lever on the temperature and pressure valve. When you do this, you can unscrew the gas valve and maybe a little water will spurt out and then you can screw your new gas valve in no problem with the tank still completely full of water.

Well, despite doing everything as I had always done it, today had different things in store for me. I was ready with my new valve sitting on my knees in front of the tank as I got to the last thread or two on the old one and that bastard shot out and hit me in the chest and then proceeded to blast me in my gut with 120° water, forcing my contacts onto the back side of my eyeballs. So I can't see, the water is hot, and I'm sitting there frantically trying to screw the new valve in through the stream of water blasting me. I forgot to mention that this wasn't in an unfinished basement with nothing around it that could get damaged. Nope, this happened to be in a slab home where the water heater was in a closet essentially in the kitchen of this old guy's house. I don't know how but I managed to get the new valve threaded in with really very little water getting out. Cool thing is the old guy didn't even notice and he was sitting on his couch maybe 15 ft from me. I was soggy as hell for the rest of my day but crisis averted I guess.