Wrench Tosser
A person who manages to screw up or break most anything he/she touches.  Usually when working on things like cars, carpentry projects, or most anything that involves tools.  

The term "wrench tosser" is in reference to someone dropping a wrench into a turning engine or gears and more than likely causing damage.
Don't let Steve work on your car, he'll screw it up worse than it is!  He's such a Wrench Tosser.