Summary: the manufacturer created the equivalent of a check engine light, except it's a mysterious code for different 'fault' conditions. Like a check engine light in a car, the user has no idea what is wrong and sometimes it's nothing important at all, others it can be serious. Unlike a check engine light, the machine won't run while it's lit. In this case, dumb stuff like 'you filled it too high' cause the machine to shut down, the owner to have no idea why or how to correct it, and a technician has to be called for a service call to scoop out the excess and reset the code for big $$$.
Basically McDonalds signed a deal with a milkshake machine maker giving them the authority to screw franchise owners every day, forever.