I run G and Z scales.
I have some locos with kind of, well, nasty motors, mostly old Marklin 3 pole motors in Z scale. Quite often my TCS decoders will not run these locos well, and often the loco goes off into full speed or becomes uncontrollable. I've normally solved this with a decoder reset. So I have seen this erratic behavior, and I either change motors or locos or get a different decoder like a Zimo.
I've only shorted the motor output of a decoder once... I ran the motor so hard (way out of it's limits) that it overheated, and locked the rotor. It sat cooking for a good while and burned up the output section of a G scale decoder. I took that situation as my fault.
Greg