Fortunately none that bad for me, ha ha!
I had an interesting experience last year though. I dropped my final passengers for the night at Long Wharf and heading back to the marina I picked up a plastic bag around the shaft. (I did not know it at the time). There were no signs of any problems and I was cruising at regular speed across the harbor. There was no drop in RPM or vibration or anything. I approached the slip as I normally do, had the perfect angle, dropped intro neutral, then finally into reverse to prop walk parallel with the dock. Except this time reverse had no affect. The rear of the slip was approaching so I poured on more throttle. I finally had full reverse with no affect. Fortunately I was not going that fast but I hit the rear of the dock with a jolt.
The bag had wrapped in a way that it did not affect the shaft in forward but she bound up snug in reverse. A diver looked at the prop and shaft the following morning to discover the bag. It served as a good reminder to approach a dock as fast as you are willing to crash into it!