If you saw a truck billowing smoke at a light you would probably just roll your eyes. There go those coal-rollers again! But sometimes, a diesel billowing smoke is not a product of a bored county boy with more money than sense, but instead an engine eating itself from the inside until it is thoroughly cooked.
To understand what is happening here, remember the basics of combustion: Fuel, air and spark. In a gasoline engine, the spark fuel and air are all controlled by various systems that can be shut off by the turning the key or letting off the accelerator. A diesel, however, uses compression instead of spark to ignite the mixture and some don't regulate airflow either. It is these engines that can be at risk of runaway.
If an outside fuel source is introduced, like a blown turbo seal dumping oil into the air intake, the engine will become uncontrollable since cutting the regular fuel supply does not starve the engine of any of the three components of combustion. The result is an out of control engine that will only stop after consuming all of the fuel or catastrophically failing mechanically.
While it isn't as common as it used to be due to better engine design, all it could take is a bad turbo seal and you too could have a big problem. In event of a runaway, Auto Evolution says the only way to stop it is by cutting the air supply either with a CO2 fire extinguisher aimed at the intake, or physically blocking the airflow. Of course, extreme caution is required when approaching an out of control, smoking, possibly oil starved engine revving at high speeds. Hopefully it will never come to that.