What happens if your lucky horseshoe falls off the wall and breaks a mirror?
you clean up the glass and go on with your life
Is there a punchline for this? 
Not being superstitious, I am inclined to agree with jesleigh, in that the only thing that would happen is the inevitable mess.
However, according to legend, and information found at this website,
From this we can derive that an unaffixed horseshoe is no longer lucky, thus the broken mirror will bring seven years of bad luck, despite the fact that it was caused by a supposed lucky object.