Broken ankle
From QWiki
Gameplay feature introduced in Kombat Teams.
In QuakeWorld when your landing speed is less then -650 (happens if you land from a high place), "hard land" sound is played and you also receive 5 damage points.
What wasn't present in original QuakeWorld and what Kombat Teams mod added was a behavior which made it close to impossible to continue bunny hopping in case of falling from a high place.
Later mods added Fallbunny setting which allowed to disable this new behavior.