Once saw a Piper Saratoga getting a runup and final check after maintenance. The engine was running and the mechanic was in the seat looking at the panel, looking at a clipboard, looking at his elbow... hell, I don't know. All I know is that he wasn't looking out the window. The aircraft crept forward, unnoticed. I believe he looked up just as the prop began to dig into the side of the parked Jet Ranger, amidships.
Nobody hurt. Everything insured. Very cool to examine the damage and see all those things (evidence of who was doing what and when) you always read about in the NTSB reports...
Dave