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Jumpbug is, next to the Edgebug, the most famous technique of falling down a pretty long distance without taking any damage and/or dying.
Error code: 126
The engine applies the fall damage if the check if the player is on ground succeeds. Such checks are done at the end of movement frames. When you perform a jumpbug, you start in mid-air and end in mid-air in one movement frame, while falling on ground and jumping happens in-between. The game never knows if you were on the ground or not, never applying the falldamage.
You have to fall down, duck (hold the duck key), jump and release duck at the same time while you are close to the ground. When you unduck, your full-sized bounding box should end up within 2 units from the ground, or otherwise you won't be able to jump on the very same frame which is required for the bug to work.
Jumpbugs can be perfomed on any solid surface (ground, another player, solid entity), from any height, with any vertical and horizontal speed, but the more vertical speed you have the less likely it is that you will "fit" into the 2-unit window. The default maximum falling speed is 2000 ups, which means that on default settings at 1000 FPS there will always be at least one frame for unducking and jumping.