From The Urban Dead Wiki
List of Zombie Hunter skills
Current Headshot Message for zombie: You took a Headshot from <name of killer> the Zombie Hunter! You will need to spend an extra 5 AP to stand up. (month-day hour:minute GMT)
It should be noted that it is not required that you kill the zombie with a firearm to achieve a headshot, all that matters is that you deliver a killing blow.
It does not matter how you kill a zombie, so a zombie can be headshot by reducing its HP to 0. Punching, for example can still cause headshot, even though no shooting has occurred.
Headshot was originally added to the game by Kevan in order to combat the exceptionally fast progression of zombies up their skill tree. By removing XP from the zombie affected, the idea was to slow the zombies down and balance the game for the survivors.
When Headshot dealt XP damage, some players felt that the skill was a form of griefing; being affected by Headshot made the game difficult and frustrating for zombies, while giving no direct benefit to the person using the skill beyond inflicting XP loss on the zombie and therefore slowing down its levelling progress.
Some of the players who went On Strike considered that the change to Headshot did not go far enough, or even made the game worse. This feeling has largely gone away; it is in fact now considered strategically ineffectual in and of itself to kill a zombie, even if using Headshot.
Basis for Headshot
It has been claimed by some on this wiki that the new Headshot penalty was a result of a suggestion here on the wiki. The closest suggestion was that on the 10th November under the heading Headshots remove AP, not XP. The suggestion is as follows:
A comment by Squashua about that suggestion states:
It is currently unknown if this suggestion was the basis Kevan used for the change in Headshot.
The suggestion received 14 Keep votes vs. 10 Kill votes and based on the Suggestions Rules was placed into the Undecided Suggestions category as it did not receive 2/3rds of the total vote.
Headshot in Monroeville
Within Monroeville, Headshot originally worked identically as before. However, after a suggestion was made to make the city more like George Romero's original, headshot was changed on March 28 2008. Any zombie headshot within the city could no longer stand up, and were effectively removed from the game. This perma headshot remained in place until June 13th when the system was reverted. Previously headshot zombies were able to stand again.