From Team Fortress Wiki
A telefrag is a type of death that occurs if a player is too close to a Teleporter exit as an enemy player uses it. Any players that occupy the same space as the teleporting enemy are instantly killed and the teleport completes successfully. Multiple players can be telefragged at once if they are all inside the teleporting player's bounding box.
Telefrags deal 1000 damage and are indicated with the "unspecified death" skull and crossbones icon, with the person teleporting being credited with the damage and the kill; since telefrags are relatively rare and there are no signs as to one having taken place, many of them go unnoticed by bystanders.
There are only two ways for a telefrag to occur: either a player is too close to an enemy teleporter exit as an enemy uses it, or a player is too close to a friendly teleporter exit that is then used by an enemy Spy. It is not possible to telefrag allies unless friendly fire is enabled. Players can be telefragged even if they are ÜberCharged, under the effects of Bonk! Atomic Punch, wielding the Dead Ringer, or under any other damage-reducing effect.
A particularly devious Spy strategy involves sapping an Engineer's teleporter entrance and then standing on it. If the Engineer is careless and is standing too close to the exit while removing the Sapper, the Spy will instantly teleport the moment the Sapper is removed, telefragging the Engineer and allowing the Spy to sap the Engineer's remaining buildings at his leisure. This tactic can also work in reverse; careless Spies sapping an exit may wind up being telefragged by an Engineer repairing the entrance. Similarly, Engineers can place their Teleporters at chokepoints and telefrag enemies as they walk over.
- Telefrags are now credited to the player teleporting, not to the Teleporter Exit.
- Fixed Spies not being telefragged when standing on an enemy teleporter exit with the Dead Ringer enabled.
- Fixed server log not listing telefrag kills as weapon "telefrag"
- Fixed defense buffs reducing crush and telefrag damage
- Telefrags exist as a holdover of the old Quake/Team Fortress engine. In some maps teleportation was required to complete objectives, but a player teleporting into an enemy would result in them getting stuck inside each other. Telefrags were the solution to this problem, as the alternative - teleporters not working if an enemy is standing on the exit - was considered exploitable.
- The term "telefrag" itself was coined by the community of id Software's earlier title, Doom. "Frag" is the word used for the point awarded for a kill in the Doom and Quake series.
- From the game's release up to an unknown date, telefrags were credited to the Teleporter Exit, not the player teleporting. Thus, one could have had a Deathcam that displays, "You have been killed by the Teleporter Exit of X"/"You have been killed by the Teleporter Exit of the late X".
- As the damage caused by telefrags is credited to the teleporting player, any abilities that charge through damage dealt (such as Buff Banner rage) will be filled with a successful telefrag.