Zero Motorcycles has recalled two models of their electric motorcycles. The US National Highway Transport Safety Administration (NHTSA) document number 12V-455 outlines defects which could effect up to 315 of the electric vehicles.
The document describe the issue as being related to a "motor encoder". Typically 3 phase electric motors driven by inverters need to know the rotor's position so the inverter can switch on the approriate electromagnet. It appears that the vital information from the sensor can "drift" about somewhat, and reports the position with slight inaccuracies. It seems that the motor controllers, on the effected machines, decide that the input information is no longer to be trusted and then justs switches off.
Simply turning off the motor is considered dangerous as the bikes could stop in the middle of an interection, exposing its rider to a dangerous situation. The NHTSA document even goes so far as to say the fault could cause death.
Zero Motorcycles is currently changing out the motor controllers at no charge to their owners. If you own a 2012 model S or model DS then phone Zero on 1-888-786-9376