Edgebug is a glitch in the engine that allows the player to fall down a long distance without taking any damage, by hitting the exact edge/brush of a certain block. It is easy to do on stairs or other broad edges.
In the case of Portal 2 and some of its mods, it allows the player to keep a button activated when there is nothing on it.
https://www.youtube.com/watch?v=FqeA_tHeU08
TODO: To make a similar video of showcasing edgebugs in multiple games, or find a better one.
It's hard to pinpoint the exact moment edgebug became a thing among players and who exactly brought it to the masses (assuming, of course, there even is a single person who discovered it first). After all, the glitch is a lot easier to trigger accidentally, unlike the infamous jumpbug.
Originally, the edgebug was known within the kreedz climbing community at least as early as June 23, 2005.
On April 21, 2006 this knowledge was shared on HL2DQ forums by the user WHO DAT BE, HUH? UH... I DUNNO; MAYBE IT'S LOST TO TIME :c in a thread titled "HL-Engine: Edgebug" (re-hosted on SourceRuns forums):
Hi,I m a well-known jumper of kz_maps (kreedz climbing maps) for Counter-Strike.I m not sure if you guys already know the bug i wanna present you.We kz-jumpers call it “Edgebug”. It allows you to jump down from a very high place to a lower level without loosing any HPs. It’s a bug in the HL-Engine and although we are only using this in our kreedz-record-runs it should be definately possible in HL also. Maybe this trick could help to save some time in a HL Speedrun.
But it was already pretty much known by that time.
It was considered impossible in Source. When did someone disprove it?
https://www.youtube.com/watch?v=J7X4odjoFck
A more in-depth explanation please. With code snippets and stuff.
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 an edgebug, you start in mid-air and end in mid-air in one movement frame, while sliding off of an edge happens in-between. The game never knows if you were on the ground or not, thus never applying any fall damage.
It doesn't matter if you are ducked or not in order to perform an edgebug.