As Deaf Smith stated, a common denominator in shooting accidents during tactical training involves an instructor loading up to leave the training facility, then forgetting to unload upon returning. It's inexcusable. First, someone should have checked weapons at the resumption of training. Second, in many academies, they no longer use any firearms capable of firing live ammunition whenever others are playacting the bad guys. Entry exercises with stationary targets is usually a live fire exercise. Decision/reaction training against fellow officers should only be done with training guns, never duty guns.
Int'l Assoc. of Law Enforcement Firearms Instructors