He will tear the mattress tags off your mattress.  He will cut ahead of you when boarding the plane and fill up all of the overhead compartments near your seat.  He will leave the iron on.  He will leave you garage door open and the gate unlocked.  He will clog up your toilet and steal your plunger.  He will drink red wine and rest the glass on his thigh while seated in the center of your white leather couch.  He will borrow your car and not chip in for gas.  He will leave the freezer door open and cause all your Ben and Jerry's to melt.  He will eat crackers in your bed and leave behind crumbs.

But worse of all, he will steal your rug-- the one and only rug that really held the room together.