Floors that instantly kill you when you land on them! Very common in zdoom mods, rather straightforward to implement in MBF, and I recently found out it's doable in Boom. Most cl2/9/11 mapsets with "you fall, you lose" mechanics use one of the following techniques:
1) -20 dmg floor: Slow and annoying.
2) Linedefs that trigger voodoo crushers or telefragging: Easily broken in multiplayer.
3) Invisible enemies that melee you to death: Infinite height problems.
4) TOUCHY enemies that explode on collision: Only works in cl11, cl11 is bad.
It turns out that false floors created by Boom action 242 block certain line of sight checks, including those used by A_Explode. If you place a constantly-exploding object in a "deep water" sector, the player will be dealt large amounts of damage if they are within range, but only if their z-position is below the fake surface!
Here's a small example wad to demonstrate the effect: rbkz.net/exp/deathfloor.wad
Here's a map I made yesterday which incorporates it: rbkz.net/doom/pwacP.wad
The effect is so simple that I have to assume the LoS behavior in 242-affected sectors was intentionally designed to facilitate things like this. There are probably Boom wads out there that have done this already?
This just makes it easy on the average player, because when they fall down, they get close to an instant restart: Thanks ribbiks