The thread on the sinking 200' yacht reminded me of something I saw about 20 years ago working on a tour boat, where a 50' motor yacht pulled away from the Blake Island Marina (near Seattle, WA), and one of the family members who was handling lines set a coiled nylon 3/4" line attached to a cleat on the stern rail. I watched from shore as the line slipped off the railing and dangled in the water. It was fine, until the captain/owner reversed the prop on that side to negotiate the turn out of the marina, which pulled the line into the prop, pulling the line tight... and then, just like a winch, pulled the prop and shaft far enough to the side to flood the boat. They got it back to the dock, got all 7 pax off, plus the dogs, cats, birds, but even with a gas-powered crash pump from our boat trying to slow things down, $200K+ (at that time) of yacht was on the bottom of the marina in less than 10 minutes. Coast Guard, State Rangers, County Sheriff, EPA, etc. all got involved, boat had to be raised and removed, big fines for (what seemed like a trivial amount) of oil/fuel released, etc. If the person handling that line had just hung it on the provided hook, 6 inches from where they set it down, there'd have been no problem at all. Small Mistake, Major Consequences... what have you seen in this category?