why do heaters always fail in the "on" position?

[ScubaSteve]

To answer your question... The bimetallic strip activates the switch into the ON position and as it switches over there is often a small arc between the two contacts when they're only a short distance apart. Sometimes this arc can actually cause the contacts to weld together, thus leaving you with a heater that is permanently on. This usually happens as the bimetallic strip begins to fail and the switch switches over slowly or the contacts become worn with age.