Fall damage

From TF2 Wiki

When you drop from or jump off a ledge, how far you fall depends on how much fall damage you will take when you land. Fall damage is accompanied by a distinct 'crunch' sound.

Crouching in mid-air and hitting the ground while crouched will cause you to fall further and increase your fall damage. This is because crouching in mid-air in Source moves your "legs" up (causing your bounding box to be smaller and higher off the ground). 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

See here