So December 4th I sold .75BTC at 1200 for $830 on Coinbase.

Or so I thought.  The next morning I received an e-mail 'You just received .75 BTC'.  I checked my Coinbase account and sure enough I got back the exact amount I sold the night before.  Now when I check the Buy/Sell history it still shows up as a sale, but if you click on the actual transaction the status is cancelled. 

How is it fair that they cancel your BTC sale?