My favorite downwash screw-up:
I was ferrying an H-53 (42,000 lbs of mighty moose muscle) down to Norfolk VA from Stratford CT when the weather went sour. We were down to 300 feet and couldnt reach anyone, so we stopped at Accomack County airport, a nice little 2500 foot strip, so my CP could telephone ahead. We just passed at a low hover down the runway to the mid-field shack, where my buddy went in with a pocket full of dimes while I idled by the gas pumps. He was quickly seen back-peddling out of the office with the airport manager pointing a finger in his chest, yelling at him is words that could almost be heard above the two 4500 HP engines idling on the helo.
Why was the airport manager mad? It turns out that he had just finished spending all morning prepping the airport for a small air show to be held the next day, and he had placed hundreds of bean bag lights, small signs for parking and guide ropes and stands just off the runway. When I'd hovered down the runway, I had blown literally every one of these things into the woods and swamp around the airport!