Friendly fire

a weapon or weapon system that (usually unintentionally) harms friendly or allied forces. More than one person has drawn attention to the irony of this term by pointing out that firing upon anyone, allied or not, is generally considered a pointedly unfriendly act. Casualties caused by friendly fire are sometimes referred to as blue on blue casualties, due to the common practice of representing friendly forces with blue symbols and enemy forces with red symbols.

"You killed Church, you team-killing fucktard!"

- Cpl. Lavernius Tucker, Red vs. Blue

In Battlefield
Titles in the Battlefield series generally impose penalties for friendly fire, such as deductions of points or kicking from the server.

Most titles make provision for the server administrator to reduce or deactivate the effects of friendly fire. While this makes for a less "realistic" environment and allows for ridiculous tactics (such as the indiscriminate employment of explosives in hotly contested areas), many agree that it is a worthwhile trade-off in order to decrease the effect of and satisfaction given to those who would maliciously cause grief by killing their own teammates.