Sabotage of aircraft is easy for a disgruntled employee.
Two cases where I was employed, so there must be many more. (It wasn't me!)
1. Many years ago a guy that was about to be sacked, loosened all the fasteners that held the black boxes in the radio rack of a BAC 1-11. Several days later the aircraft had a series of unrelated failures and the problem was found.
2. Someone with a grudge cut some wiring on an A300 that was undergoing a "C" check. Some problems were only found during a test flight.
How do you prevent it?