One of the most disturbing stories that I don't hear mentioned is of Sean Baker, who was working undercover for the USAF as part of an attempt to gauge the abuses at Guantanamo. Baker was beaten by the guards so badly that he got a serious brain injury.
"In January 2003, Baker was ordered by an officer at Guantanamo to play the role of a prisoner in a training drill. [...] The soldiers in the reaction force were operating under the impression that he was a genuine detainee that had assaulted a sergeant. [...] Although Baker shouted out the safeword ("red") he had been given to stop the exercise and stated that he was a U.S. soldier, the soldier continued beating Baker's head against the floor and choking him. Only after he ripped his prison jumpsuit in the struggle, revealing that he was wearing a battle dress uniform and government-issue boots underneath, did the beating stop."