The Bulldog had (has) a rotary mag switch, which controlled both mags, rather than two separate ON/OFF switches.
I can remember one of the Standards Squadron examiners telling me that it was possible for the metal contacts in the switch itself to malfunction such that at "OFF" the mags, or at least one of them, wasn't grounded.
The mags could still be grounded separately as normal, because the switch used different contacts inside the switch body to ground them at the L/R positions.
Unless a ['Both mags OFF / "Drop & Stop"] check was done, you wouldn't know the switch contact had failed.