Modrons can and do go rogue, often as a result of spending too much time alone, attempting to reconcile too many conflicting orders, or from exposure to Chaos.
While rogue, they retain all their powers, including their command over lower-ranked modrons, but are no longer subject to orders from higher-ranked modrons.
On death, they return to the energy pool on Regulus to be reborn as a (non-rogue) monodrone, the same as any other modron. This gives them a sort of mortality.
Rogue Modrons first appear in the AD&D1e Monster Manual II, are developed across the AD&D2e Planescape setting (notably in the Planes of Law: Mechanus supplement), and one named Nordom is a recruitable character in the computer game Planescape: Torment. Rules for playable rogue modrons first appeared in Dragon magazine, issue #235.