IIRC someone implemented this attack (buying or selling small amounts could move the price on BitStamp and Coinbase would offer to buy/sell large amounts at that price), notified Coinbase, and now it's fixed.
I think they were using the "last price" instead of the "weighted average", or some stupid mistake like that, and obviously without any sanity checks. Meaning someone could manipulate Coinbase's price by placing a tiny buy order for a very high price or a tiny sell order for a very low price.
