Head Bump

Inflicts damage when the player hits their head on the ceiling. Damage is 0-10% of a heart, depending on how hard you hit the ceiling.

Helmets reduce damage by 25% but lose some durability.

Why would you want this? I originally made it to stop players from digging 2-block-high staircases, because now they'll take damage when they hit their head on every step when they come back up. You can still dig ugly staircases - this mod doesn't prevent you from making 3-high staircases, or 1-wide staircases, or stairless staircases - but it's a start. It also prevents the "feature" of traveling very fast by jumping in a 2-high tunnel.

I have another mod (Starvation) that you can use to discourage stairless staircases, and I'm planning on writing a mod to discourage 1-wide passageways. Ultimately, I want to make it so the optimal staircase for players to dig is also one that looks good. I don't want SMP players who care about aesthetics to be crushed by players who just optimize for becoming powerful as quickly as possible.

A related mod I'd recommend to discourage ugly strip mines is Swan's Law by SwanX1.