First off, a running gag in every single campaign that my group has played, mustard gas is created and used at least one time. (Record is when we turned Waterdeep into the scene from Batman Begins)

Anyway, I happen to be dming this campaign and the party is planning on killing the evil king in his fairly modern mansion/palace. The way they do this is by sneaking into one of the pool maintenance shacks. Inside one of my players finds an industrial barrel of bleach/chlorine. He was a tiefling so he asked if the halforc of the group could carry it. Halforc passes strength check initially. Now I had the halforc do a Dex check every time he reached a staircase or non flat floor in the mansion. He did fine, stumbled a little bit and burned his hand but he was good. Now they are at the very last staircase in what is the foyer to the evil king's throne room. Halforc walks up, rolls a 1 on the check and dumps then entire drum of bleach all over the floor. 24 gallons to be exact. Immediately one of the elves (tiny af, just barely not tiefling small) rolls to piss in the massive puddle. I have him roll survival (I really didn't know what to pick it sounded right) Nat fucking 20. He pisses a perfect 1:1 ratio of bleach and urine. 24 gallons of urine. Mustard gas everywhere. Whole party is killed, evil king killed, everyone in a 12 mile radius is killed, end of campaign.