"Friendly Fire" is the second episode of the sixth season of Numb3rs.
Charlie and the team investigate a shootout involving a bank robber, but when the scientific evidence doesn't match the stories suspicion falls on one of Don's mentors, who is due to retire.
Title
The military scenario wherein a unit or troop is killed by weapons discharge from their own side (including allies). With modern forensics and ballistics there are presumably far fewer such incidents, pro rata, than in the past. History offers numerous examples of unpopular officers caught in such, as well as genuine accidents.