I'll start :
leaving a towbar on a nose wheel then starting the engine.
I'm that guy, only worse. I once
nearly started my Mooney up with my Redline Sidewinder powered tug still attached. (see below) If it weren't for an observant A&P working on his own plane on a tie down waving me off, I would have had a very, very bad day. There was never any danger of me hurting myself, or anyone else as the plane never would have moved a foot. The prop would have hit that machine, damaging the tug, the prop, possibly the nose truss and linkages and maybe requiring an engine tear down.
So of course, most are wondering, how can you possibly miss that thing on the front of your plane?? I don't really know myself, but as I walked the short 15' or so from closing the hangar doors to walking around behind the plane to the passenger side I missed it. Likely thinking ahead to the flight, or who knows, I can't remember. Once you are behind and walking up the right side to board the plane, you can no longer see the tug. Once in the plane from any position you can no longer see the tug.
Fortunately another pilot and plane owner as well as A&P, was watching the dumb ass in motion. He keenly observed be closing up, strapping in, running checklists and finally switching on the beacon and starting the boost pump. At this point he waved me off. I never cranked the engine.
I have since created a new procedure. I open the hangar with my keys (that also have the plane's ignition key attached) and then place the keys in the place where the tug goes when I'm out flying. This requires me to go get the keys when I put the tug away. This system is working well now, but even this took some work to turn into a routine. I was so used to putting the keys back into my pocket, that I really have to work and not doing that.
Anyhow, I'm on the other side of this discussion. My name is Dave and I'm a recovering pilot dumb ass.