Is there any evidence that Coinbase actually did that? My read of the Coinbase founder's post up-thread was that they purchased the Bitcoins on the user's behalf at the time he entered the order, but a server failure prevented them from being transferred to the buyer's wallet. If they had given him the current market price they would have to purchase additional Bitcoins and eat the cost. (Which, reading the recent edit, seems to be exactly what they're doing.)

I agree this is not good for Coinbase, but I am also far less surprised or indignant than many posters here seem to be. This is what the early stages of most markets look like, with fuck-ups and manipulation galore. Read a book on the history of the NYSE (and later, Amex, which was called the "Over the curb" market because it consisted literally of people shouting orders at each other on a street corner in NYC) - this is far milder than the stuff that went on then. At least Coinbase seems to be exhibiting incompetence rather than the naked greed and malice that characterizes many other nascent markets.