You cannot jump down from a platform if it is at least a certain height above the ground. This mechanic is a major annoyance in many maps (including underwater maps where fall damage doesn't exist) and benefits absolutely nobody. The inconvenience of taking fall damage is pitifully small compared to the inconvenience created by the game's attempts to protect you from fall damage. Please, just let us jump down and take fall damage. It's probably not even that hard, either, just find the variable that corresponds to the maximum permitted jumpdown height and set it to 65535 or something.
If there were no jumpdown height limit to begin with, and someone posted a feedback thread saying that they should add a jumpdown limit so people don't accidentally commit The One Who's Touched the Sky when jumping down the Kerning City crane when they have less than 20 HP, do you think that person would be taken seriously or do you think they'd be laughed out of the room?