Source™

Look, sometimes Source is just a little stupid! Sometimes, a little more than a little. Stuff you'd never think would break in a million years can break. Something may work fine in one game, yet be broken in another for no discernible reason. Sometimes "features" have been interpreted as bugs!

Source is past it's days of being a viable engine for completely new games. It's still the obvious engine choice for mods of games already running Source. On a related note, any stray features that Valve did not use themselves are probably non-functional.

The biggest mistake a Source Ents editor can make is assuming. It's best to not baselessly assume things work in Source. When in doubt, test it out!

Accelerated Back-Hop
The ABH is a Source bug often seen as an actual part of the engine itself due to being so iconic. The accelerated back hop is a bug discovered after Valve attempted to patch the (sort of equally buggy) bunnyhop. When the game thinks the player is bunnyhopping, it tries to slow them down by subtracting speed from their forward velocity. When the player bunnyhops backward, the subtraction only causes their "backward" velocity to increase. This bug is now the bread and butter of HL2 speedruns.

Upward ladder jumping
In Day of Defeat: Source and Half-Life: Source, you can jump off the top side of a and briefly fly into the air. Counter-Strike: Source and the Left 4 Dead games don't have this issue, suggesting it was intentionally included in DODS and HLS.

ambient_generic
has many bugs, but perhaps the two worst are:
 * Sounds may still be audible outside of the Max Audible Distance.
 * Setting the volume to 0 makes sounds quit playing completely. According to a code comment, this is completely intentional yet was considered a bug by VDC editors. (Really, that's correct. It should simply make it silent, not halt it entirely.)

Alternative names

 * Valve™
 * Alien Swarm™
 * Havok™
 * Hammer™
 * Turtle Rock™
 * VDC™