A floor mirror, or full-length leaner, is a large tall pane that is especially prone to flexing because of its length. Its size makes it a two-person carry, and its habit of leaning means the base and corners take a lot of stress. Keep it upright, protect every edge, and never let it bend and a floor mirror moves without cracks.
Pro tip: If the mirror attaches to a stand, secure or detach the stand so it does not swing and torque the glass.
Watch out: Long mirrors flex more than small ones. Never carry a floor mirror flat, and keep it upright the whole time.