The State of Washington Attorney General posted an entry on the homepage telling users to be wary of credit/debit card blocking. Card blocking can be defined as:
"Credit or debit card blocking generally occurs at hotels, gas stations, restaurants or places where one can rent a car. When you use a credit or debit card before the total amount of the purchase or service has been determined, a merchant will sometimes request your bank block out the estimated total cost to ensure they will get paid."
"For example, when you swipe your card at the gas station before beginning to pump gas, the station will often request your bank put a $50 hold/block on your account. Once you have completed your transaction the final charge replaces the hold on your account. Usually these holds are replaced within a day or two."
This can quickly eat into your available balance, making your card useless for a day or two. Definitely watch out for this while on vacation or if you are near your limit!