I flew a Hughes 269B in which an engineer had applied non approved anti-abrasion tape to the M/R blades. All was fine until I flew through a rain squall.
When I tried to hover out of ground effect the thing wouldn't stay in the air. When I landed I discovered that the rain had shattered the tape thus affecting the aerodynamic properties of the blades. After it was removed all was fine. It was a cheap lesson that could have ended in disaster.