A wall mirror is a large sheet of glass that flexes and cracks under its own weight if carried flat or set down on a corner. Mirrors also need edge protection, since a chipped edge quickly spreads into a full crack. Wrapping it well and always keeping it on edge, never flat, is how a mirror survives moving day.
Pro tip: The tape X will not stop a break but keeps shards together and is a quick, cheap safeguard.
Watch out: Never carry or lay a mirror flat. The glass flexes under its own weight and can crack in the center.