I was involved in accident investigation for the Southern Pacific, but I never heard of any accident related to the signal mechanism being jammed. I do recall an accident wherein a signal maintainer turned a relay upside down when working in a signal box, which resulted in a head on accident when one train received a green aspect instead of a proper red signal, and entered a siding at speed that was occupied by another train.

Did investigate an accident wherein a crossover had been wired backward resulting in the gates of a nearby grade crossing going up instead of staying down as a railroad car entered the crossing. And yes, an automobile ran into the car.