Candy Bar Wrappers were being found in Chinook Transmission Sumps....Boeing discovered the boxes in for overhaul were stored next to an employee break area and the wrappers were being thrown into them instead of Trash Cans.
Once the Transmission halves were moved away from the Break Area....the problem was solved.
Then there was the Bell Short Shaft on the Huey....that could be installed wrong way around.
Lots of Murphy issues have cropped up over the years.