Overall, this is a big disappointment because it seemed like Coinbase was finally the first professional service for buying and selling coins. Unfortunately, it looks like it's still amateur hour in Bitcoin land -- as usual.
More than one person has noted that transactions have a much higher probability of being classified as "high risk" if the exchange rate has gone up significantly between the time it was started and the time they credit your account with bitcoins. One would think they allocate the bitcoins immediately at the time they lock in the price, but if they aren't doing that it would make sense for them to cancel the order if the price has moved too far.