Friendly fire is inadvertent firing towards one's own or otherwise friendly forces while attempting to engage enemy forces, particularly where this results in injury or death. Friendly fire happens on the battlefield...it happens in marriages...it happens in the Church...it happens in life. Wounds are sometimes caused by trusted friends and allies.
Churches are full of hurting and wounded people. Tragically, many of them have been hurt not by enemies but by friends or fellow believers. I would highly recommend a book by Ron Dunn, entitled Surviving Friendly Fire. It will help you respond in a God-honoring way!
Oh, and here's an example of what friendly fire looks like.