Source™

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 and Left 4 Dead games don't have this issue.

ambient_generic

 * "Maximum distance that this sound may be heard from. Impressively, sounds are still slightly audible outside this range."

prop_paint_bomb

 * "The value given in the FGD is wrong.  needs to be added as if it were a missing keyvalue, but having spaces in a keyvalue name breaks the FGD parser, so this keyvalue is completely unusable."

npc_alyx

 * "Determines if Alyx can reprogram rollermines using the EMP tool. Doesn't stop her from trying!"

asw_objective_triggered

 * (On the SetIncomplete input) "The display near the top left will proclaim 'Objective Complete!' when this input is fired."