A colleague a few years ago flying a Gazelle helicopter had an engine failure (almost unheard of in Gazelle). He got it safely to the ground and the engineers came out to have a look.
Started up, ran, all systems green.
Engineers looked everywhere for about an hour, then gave him a single flight auth for the 20 minutes back to base.
10 minutes later, another engine failure, another successful forced landing (pretty impressive). He was not asked to fly it the last 10 minutes, and it went by truck!
Problem: a ball of used duct tape sloshing about in the fuel tank that intermittently blocked the fuel supply in flight, but fell out and clear whilst ground-running!
One to have in the back of your mind!
Sam.