I have a main panel (unfortunately without a main breaker) and a sub panel. The sub panel is fed from the main panel via a dual pole breaker. One red and one black connection with some pretty hefty looking wire.

When I turn off the dual pole breaker (dual switch connected), the black wire to the panel is no longer hot, but the red remains hot. I see no reason why this would or should happen, except that the breaker has failed. Can anyone concur that breakers do fail in this way?

Thanks in advance.