[LONGHAIR]

I have the same one too and I hav not heard about this problem either. Mine is a few years old though and never had a problem. Maybe a newer manufacturing glitch?

Just an FYI the little loose piece is the "key" ....the groove that is fits into is the keyway. These must be using a square key. They give a little more latteral movement, which will allow you to align the pulleys over a wider range or to use pulleys with wider hubs for multiple grooves.
Smaller "woodruff" keys are rounded on the motor shaft side. They cannot fall out unless the pulley comes all of the way off. These are usually more of a press fit situation, but not always. Straight keys are more user friendly