Mistakes do happen. We had a new engine installed on a Piper PA-32 Lance. The mechanic forgot to torque the oil hose. He hand-tightened the connection. Eight flying hours later, it wiggled loose and dumped all the oil at 6000ft.
The club pilot who flew it (it was supposed to be me) was in the middle of an IFR snafu with ATC and was flying 6000ft above the airport, circling. It was easy to get it on the runway but not to fly it. Oil covered the entire front of the plane and windshield.
The engine seized on the runway as he was landing.
The mechanic fought it for months but finally had to buy us a new engine.
It was a very costly mistake.