Interesting Facts from the Killing Floor Code: Hidden Mechanics

A collection by Dr.TerV from the "Interesting facts in Killing Floor" thread on the killingfloor.ru forum (2011; the original was in the KF group on VKontakte). The author knew his way around the game code and checked the facts against three years of play, but warned there is no 100% guarantee: patches change mechanics, and servers with different settings may have different numbers. He left officially announced features out of the list.

Shooting

If you fire more than 6 shots with less than half a second between them, a spread penalty kicks in. It shows on assault rifles and other rapid-fire weapons. Shooting in bursts of 3-5 shots with half-second pauses is more comfortable.
Semi-auto mode cuts spread by 15%, crouching by another 15%, aiming down sights by another 50%. Shotguns and grenade launchers do not get these bonuses.
Each time a crossbow bolt passes through a zed, it loses 15% of its damage and speed.

Zeds

The Bloat takes 50% extra fire damage regardless of perk. This stacks with the Firebug bonus, so the Bloat is a priority target when leveling the Firebug.
The Fleshpound, like the Bloat itself, is fully resistant to Bloat bile.
A "calm" Fleshpound that keeps looking at a player for about 10-15 seconds rages on its own. To prevent that, break its line of sight, for example by stepping around a corner. The counter resets when the Fleshpound attacks, even if it misses: the attack animation just has to play. Thread members argued about this, but settled on the reset on attack.
A burning zed loses 20% of its speed every second.
Zeds try to move away from a thrown grenade, but this does not work if you are too close to them. So when a crowd approaches you can drop a grenade at your feet and run right away: the zeds will not back off and will walk even closer to the grenade. A trick for retreating or for a Berserker or Medic rush.
After a Bloat bursts you have a tenth of a second to get out of the bile.
If you shoot or chop off a Clot's head, its damage and attack reach double.
While screaming, the Siren loses 35% of her speed.
The Siren shoots down not only the player's rockets, grenades and pipe bombs, but also Husk fireballs.

Melee and movement

A melee hit (chainsaw, katana, knife, axe) from behind does double damage.
If you land on a zed at a certain speed and its mass is lower than the player's, the zed is stunned.
Jumping on a zed takes a little of its health; it is most noticeable on Crawlers and Stalkers. If a zed dies from this "stomping", it is torn apart (according to the author it depends on the zed's health; for another player the Clot was just tossed aside).
The Berserker uses the syringe and the welder faster than other perks, by exactly the melee attack speed bonus shown for the perk.
A melee attack is locked in when it starts. If you switch weapons before the hit lands, you get an instant hit from both the old and the new weapon. Combo examples: an axe heavy attack followed by katana swings; a katana heavy attack followed by a Hunting Shotgun double barrel shot; a knife alt attack followed by a 9mm headshot.
For fun on your own server: admin set kfhumanpawn jumpz 1000 and admin set kfhumanpawn mass 10000 turn the game into something like Mario.

Medic, ammo and money

The MP7M gets a full 100% healing dart charge each time it is bought or picked up. To heal a large group, use up the charge, drop the MP7M and pick it up again: the charge is fully restored.
Buying or picking up shotgun ammo also refills your 9mm ammo. That is why a level 5 Support has the most ammo at the start of a game.
The TossCash command throws money. Without a parameter it throws the default amount, but it can take an amount. With binds you can put different amounts on different keys, for example TossCash 1, TossCash (50 pounds) and TossCash 9999. More on binds in Useful binds.