The Army Air Corps were looping Gazelles at displays for a number of years before somebody thought to check on whatever passed for the Release To Service in those days. Even the POH clearly stated not to exceed 90 degrees in pitch or roll, but hey, the POH is just like a suggestion, right? I can't remember the exact fallout, but it was quite the task to track down all the airframes that had been looped, and then do the subsequent engineering tear downs to establish how many were bent, buckled or scored. All very embarrassing and quite avoidable. There was also a well known photograph of a Gazelle fully inverted (probably following a roll) against the backdrop of a large TV mast south east of Detmold in Germany, everyone admired it, but nobody ever thought to challenge the how and the why.