I remember hearing stories of a particular USAF cargo aircraft - piston engine powered - that had such a horrid engine MTBF that losing an engine somewhere on the trip from Hawaii to Vietnam was not uncommon. Since the US airbases were a regular target of rocket and mortar attacks, the crews would rather risk a 3-engine departure from South Viet Nam rather than having to stay there long enough to perform and engine change...
That would be the C-124
td, used extensively during the conflict transporting stuff from the US to Vietnam. 97 hours flight time from Travis AFB, California, to Tan Son Nhut AB, South Vietnam, and back, so it's retirement from Military Airlift Command in 1970 and replacement with jets was welcome.