Does anyone know of a case example where an MCC bucket disconnect/breaker was locked out and an arc flash occurred anyways during work in the bucket?

Yes, if I unerstand you correctly the circuit breaker was cleared and tagged in the bucket to facilitate troubleshootng a reoccuring over load condition. The electrician decided to verify tightness of the LINE side breaker cable connections. They were hot of course and his long "uninsulated" screw driver took 480 volts to ground as it touched the bucket casing. Bad practices yes. But it indeed caused an arc flash and caused a MCC trip all the way back to an upstream 1500 feeder breaker.

A problem I have seen occur is when the line side connections of the breaker are loose or the stab to buss bar connection on the back of a MCC bucket is loose the heating can degrade the wire insulation. At some times the only thing keeping the wire from shorting to the enclosure is the crispy wire insulation that is left and providing spacing between the conductor and steel. A little shaking of the bucket can cause the crispy wire insulation to fall and then contact become easier.