The nighlight electrical mystery

There was a little excitement in Big C's room this morning. There was a short circuit involving Big C's Disney's Princess Night Light which was evidenced by a loud pop and what Big C reported as 'sparkles'. There was a little smoke and at first I noticed that the outlet was no longer working when other devices were plugged in. This lead me to believe the circuit breaker had tripped, which it had.

I took a good look at the light itself (the only identifiers on it are 3163060 and FS-T707, but my wife found the product as sold at Rona). This was the evidence left behind after the incident.What appears to have happened is that a short was created between the right prong of the plug and the screw at the top of the outlet. If you take a good look you can see that the screw at the top of the outlet is melted on one side.
Odd.

Thankfully no person nor property was harmed and oddly enough, after a trip to the electric panel to flip the breaker, both the outlet and the light are working fine. I'm hoping to be able to determine what exactly caused the short. Until such a time I've secured the outlet with the kid's safety covers and put the light down on my workbench out of kids' reach.