Source™

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. The Counter-Strike games and the Left 4 Dead games don't have this issue, suggesting it was intentionally left in DODS and HLS.

ambient_generic
has many bugs, but perhaps the worst is that sounds may still be audible outside of the Max Audible Distance.

Alternative names

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