When someone uses scalding-hot tap water to wash something, and then simply turns off the faucet.
Then you come along a minute later and turn on the cold water to wash your hands, and boom, you get the leftover boiling hot water that was in the faucet. The guy before would've never run that water on his hands, but now it's running on yours, pretty much burning you.
So if you're reading this...
TURN THE TAP TO COLD FOR A COUPLE SECONDS WHEN YOU FINISH USING SUPER HOT WATER.
/rant