According to this eyewitness account, a line of train cars mistakenly unhooked and rolled away down a gentle slope, smashing through a blockade into a lot full of brand-new GM vehicles awaiting transport, destroying as many as 300. Gallery below.

According to our tipster:

The incident happened about a month ago in the Oshawa CP Rail yard which ships out the GM vehicles. One of the car men who attaches and detaches the car haulers accidentally unhooked a set of train cars without having the brakes applied. The Oshawa rail yard has a slight incline to it, so when the set of cars was unhooked from the engine it started to roll down the hill, it smashed through the stop block and then through the fence and continue into the parking lot where GM stores the new vehicles that come off the line. It is rumored that over 300 vehicles were damaged. Enjoy the pictures.

Seems like it was mostly trucks from the Oshawa Plant and contrary to the "month ago" assertion, this seems to have been quite a while ago as those are all old models.