Assuming that the pump is OFF, and the check valve is leaking and the water is comming baack to the pump causing a reverse rotation. what is the bad effect of this rotation if the pump is OFF, and no one run the pump during the reverse rotation.

Generally there is no problem if the pump runs backwards from reverse flow - however, if fitted with mechanical seal/s there could be a problem as some seals are designed to run in one direction only  - this you will have to discuss with the seal manufacturer and you must ensure that there is no attempt to start the pump while it is running in the reverse direction. 

The potential seal damage and the potential for the impeller unscrewing are certainly cause for concern.  

As long as the motor is guaranteed to remain OFF, those might be manageable risks.  But what happens when you need to start the pump to put the fluid back where it was?  Is there a means provided to stop the reverse rotation?  Will everyone bother, all the time?

Starting a motor that's already running backwards is pretty much guaranteed to cause severe electrical problems.

By way of illustration, I have one data point:  "Plugging" a small 1/15HP 115VAC PSC motor will blow a 20A breaker right out of the panel.  I don't know if the breaker exploded before launch, or on impact with the floor.  It made a surprising amount of noise.

Actually, a screwed impeller should not unscrew itself in this case.  If the motor is wired up for reverse rotation, the torque from the motor will unscrew the impeller.  But spinning in reverse from backwards flow results in the same torque direction as normal rotation.  The concern about starting up the motor while spinning backwards is very real.  We recently failed a coupling with more than a 2.0 service factor.  I believe this was the cause of the failure.  

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