Friendly fire

From TF2 Wiki

Friendly fire was an option available to servers during the Team Fortress 2 Beta. It allowed players to damage or even kill their team-mates. This made setting up an ÜberCharge quicker and also benefitted Spies as it discouraged spy-checking.

Valve removed the feature upon release as it was to cause errors within the game's code as well as harbouring potential for griefing. The low focus on accurate shooting in the game also meant that friendly fire could become an impediment if several classes attempted to co-operate with one another.

However, in normal play there are still three examples whereby a player can injure himself. Soldiers can take damage from their own rockets, Demomen can be injured by their own Sticky bombs and Grenades, and Engineers can take damage from their own Sentries.

There are also several ways to get a friendly fire kill that are still left in the game. As a Medic, you can be healing a spy that stabs a team-mate. And as an Engineer, sentry rockets can injure and even kill your team-mates. However, sentry bullets do not.

Some players report that some projectile weapons like the rocket launcher can occasionally injure team-mates but this is unclear.

Full friendly fire is still available as a server option but is rarely used in standard TF2 play.