Some medicine cabinets have a slot in the back panel so you can push your spent razor blades (not Atra cartriges, mind you) through them and into the space in the wall.
Of course this only works if you have HOLLOW stud-n-plaster walls!
My medicine cabinet has the slot and that is how I dispose of the razorblades I use for cutting things like recording tape, packing tape, stripping thin wire, etc. after they get dull.
It's safer than putting that crap in the garbage bag (which then might potentially slice through and cut the garbageman).
I wonder if, when they tear down this building, the demolition crew is going to wonder what all those rusty razor blades are doing inside the walls!!!