Banksy posted a video on his website explaining that the painting was supposed to be entirely shredded by the device.
But interestingly, the video states that it was actually meant to be fully shredded, showing rehearsals in which the full painting was shredded every time.
The auction house says the winning bidder chose to buy it anyway at that price.
"Girl With Balloon", which depicts a small child reaching up toward a heart-shaped red balloon, was originally stencilled on a wall in east London and has been endlessly reproduced, becoming one of Banksy's best-known images.
Art lovers continue to spread theories that the Sotheby's auction house itself was in on the prank. "Not really. Perhaps we are hoping for it", said auctioneer Arnaud Oliveux of Artcurial auction house in Paris, which is selling three Banksy prints next week. The 2006 piece was subsequently renamed "Love is in the Bin" and was certified by Banksy's Pest Control company, which keeps his identity secret. By all accounts, it looks like there was no way of knowing precisely what he had in store, and you can hear one of the auctioneers from Sotheby's remark that he often provides the frames to his works as part of the sale.
An unnamed collector bought the piece which, including a buyer's premium, cost £1m.