The design flaw is in the set of solder bumps associated with one of the connectors (for one of the motors). Unsuspecting people (like myself) will assume that nobody is going to design and sell a commercially-available system that will have a whole set of solder bumps that will become short-circuited by a metal surface.In this case, I was oblivious to the situation where mating the motor shield to the arduino 2560MEGA board will lead to 3 or 4 pins getting short-circuited by the shiny metal USB connector casing.
Well its a lesson in why close visual inspection of all new bits of kit and while assembling is a good idea.
You should add some insulating tape to the top of the connector.
I got my money back eventually.