The official NHTSA notice explains the problem:
"Due to a manufacturing process error, the tolerance between the engine oil pump's drive shaft and the pump's rotor was not within specification. As a result, the pump's driveshaft could separate from the rotor."
The result of that separation would be a sudden loss of oil pressure, potentially leading to complete engine failure, and thereby increasing the risk of a crash--not to mention a very expensive engine replacement.
The recall affects only vehicles manufactured between July 19 and September 11, 2012. BMW will soon be notifying owners of the issue this month, and will offer free replacement of the oil pump.
If you own one of the affected cars, you should not drive it at all until it has been serviced. There may be no warning before the engine is irreparably damaged. Owners can contact the National Highway Traffic Safety Administration Vehicle Safety Hotline at 1-888-327-4236, or go to the NHTSA's SaferCar website for more information.
2013 BMW M6