I've tried googling for this story, and I can't seem to find it.  I thought I had it saved in my bookmarks, but apparently not.  I'm not positive, but I think it takes place around "In My Time Of Dying" and "Everybody Loves A Clown".  Sam and Dean are at Bobby's and Bobby is working on a vehicle, hoisting the engine.  Suddenly, the chain holding the engine snaps and hits Sam in the neck.  Anyone recognize this one?

EDIT:  I found it! Ripple Effect