I was looking at an FAQ for some small appliance that runs or
recharges from an AC adapter, a black box that plugs into the AC and
lowers the voltage to 12 or less, and maybe changes it to DC.

And it said this was safe, and I too have been assuming these were
safe, but...

Has anyone heard of a short in one of these adapters that sent 110
volts to the thing that was supposed to charge or run off the
adapter???

That would be unsafe, if it happens.