Coasting AC induction motors offer 2 very real risks as mentioned by others.
1) Residual magnetism. Depending on the design, this can be very substantial. So much so in fact, that island mode wind generators can take advantage of it to enable self-excitation systems (Google that and you will see). They need caps to charge it up high enough, but without the RM the caps would have nothing to work with.
2) PFC or Starting capacitors. If the caps are still on-line when the motor is disconnected and coasting, they will maintain the excitation current and allow the motor to continue as a generator.
So a good rule for a millwright (or anyone else for that matter) who is NOT familiar with the magnetic hysterisis properties of that particular motor's core laminations, or does NOT know if PFC caps or Starting caps are connected, it to ASS-U-ME that there is "potential for potential" on that coasting motor.
