Three things come to mind. Each was the result of failing to pay attention to what I was doing.

Years ago I decided to swap an Airtronics radio for a JR radio while at the field. No big deal. The evening wasn't half over, and I had plenty of time. I neglected to check all of my controls. For whatever reason, I checked the rudder, elevator, and throttle. As soon as I took off, I noticed a slight roll to the right. I pushed the stick left. Bigger roll to the right. I pushed the stick further left. Nice hole in the ground. Whoops...

It must have been earlier in that same year, because that plane was still in one piece. I was carrying it back from the runway on a windy day. I had the plane in one hand. Felt it start to slip because of the wind. I grabbed the prop. The plane kept slipping, and I re-discovered that an APC prop makes for a fine steak knife. Models look better when bloody, right?

I few years ago I was hooking up a brushless ESC. For whatever reason, I was trying to work quickly. I used some Mickey Mouse method to separate the bullet connectors as I was getting the motor rotation right. Unsurprisingly that didn't work to keep the exposed bullet connectors apart, and I got to see the magic smoke flee the ESC.