Don't tow your racecar without checking the presure in your trailer tires.

Don't prime your new race engine without verifying the oil filter is in place. (Can you say "mini exxon valdez".)

Don't hold the negative clamp on the timing light while someone else pulls the trigger. You make an excellent ground.

And lastly, an oldie but a goodie,
Always remember and never forget to either set your parking brake or put your manually shifted car in gear while at the drive-in movies equipped with "hang on your window speakers with steel cable so you don't steal them feature".