It is, in theory, possible to double-pay an unconfirmed transaction. This risk is always present when accepting unconfirmed transactions, especially large amounts. In many cases, however, this risk is insignificant, such as with dining and low-priced goods. In transactions using wallets which, like BitBucks, include the services of administering the private key and signing transactions, the risk of double spending on unconfirmed transactions is greatly reduced.