Fall damage

From TF2 Wiki

Fall Damage is a type of damage that only occurs when a player falls from a certain height. The higher the player drops, the more fall damage the player will take. Fall damage is accompanied by a distinct 'bone crunching' sound.

If a player was to crouch in mid-air and continue to stay crouched until his landing, the player will take more fall damage than if he was standing upright. In the Source Engine, crouching in mid-air moves your "legs" up (causing your bounding box to be smaller and higher off the ground). Thus there would be more distance between the character and the ground which would increase the amount of fall damage. However, if you crouch-walk off a ledge and then stand in mid-air before hitting the ground, you will decrease your fall damage.

Some areas of maps which are out of bounds will also instantly kill you with fall damage when you hit them, even if a fall of that height would not normally kill you or if you negate your falling speed with a double jump.

Fall Damage Chart

Original Source