In steel mills, the molten metal is passed through a series of rollers to mold it into the right size. The steel is at 1200°C (2200°F) or even hotter. Occasionally, something will go wrong and you get a "cobble."
A cobble will occur when there is a roller malfunction, the line of steel deviates from the roller path or, as mentioned above, the end of the steel splits. All of a sudden the continuous roll of steel will come to an abrupt halt although the steel behind is still being pushed through the working rollers at speeds which can reach up to 30 mph. The steel at the front of the line has nowhere to go, the pressure builds very quickly, the material begins to coil up and then all of a sudden it will flick into the air creating enormous loops which have been likened to a “light sabre”.
That's dangerous. And it happens more often than you'd like to think.
You can see more videos of steel cobbles at Boing Boing.