Well, if the pump flow rate is higher than that of the main, then you'll be pumping water away from your neighbours and/or risking the ingress of impurities at points of negative pressure.
You would also aotumatically qualify for entrance into the National Stupid Competition.
At the risk of being accused of harping on about this, my point is that the OP didn't ask about the risk of whatever could happen when pumping the mains, but asked what would happen if he did it.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local