I guess I've only done one really stupid thing...was with my old 790 during the summer of 2009.
I was finish mowing the yard and stopped to pick up some branches and other small stuff. As the mower was on, I shut off the engine and left in in gear (790 is a gear tractor). Picked up the branches and carried them to where I pile that kind of thing.
Went back to start the tractor to idle it for a few minutes before I started mowing again (since I had a few more fallen branches to carry off). So, standing along side the 790, I knocked the lever into neutral...and there she went...slight slope (maybe 3-5 degrees, if that), but enough to get it rolling. My immediate thought was "Oh Sheet!!!!".
The machine rolled into a wooded section at the front of the property line and hit a small tree which, fortunately, stopped the tractor (otherwise, it would have rolled down an embankment on to the road).
Obviously, I hadn't set the parking brake.
Well, it could have been worse...I could have been run over by the rear wheels and RFM (even if not running, that would have done damage to my body). It could have rolled down that embankment into the road (which would have ruined a passing motorist's and my insurance company's days).
Gotta admit...I really felt like an idiot after that episode.