I punched myself about as hard as I could possibly punch myself in the chest trying to break the caliper bracket bolt doing my front brakes one day. I think I may have bruised a rib, it hurt to laugh/cough/sneeze for 2 weeks.
Also, not relayed to modding, but my first mt trans car. I parked it outside of my work, went inside and was working for about an hour. My dad comes into work, family business, and asks if I intend to leave my car there all day. I forgot to set the e-brake and wasn't in gear. It slowly drifted into the middle of the parking lot.
Left my jeep running overnight one time too. Late one night after work, was going to go out, ran inside to get something and passed out. My dad woke me up and asked what I was doing up so early that my car was running.