[Mr. Zero]

One thing I just quickly noted is that you checked IsClientConnected && IsClientInGame.

For a client to be in game they must be connected, so you can cut the IsClientConnected out if you are checking for IsClientInGame.

Have you checked that the hill in Dark Carnival map 1 (L4D2) doesn't trigger the fall damage event?
The hill uses a trigger_multiple with OnStartTouch !activator IgnoreFallDamageWithoutReset 10. Not sure if stops the player_falldamage event.