Before you even pull the flip-flops out of your suitcase for your week of vacation, your available credit may appear as though you've already paid for the whole trip.
Blame it on credit or debit card blocking -- the completely legal practice of merchants blocking off what they assume will be the entire charge for your hotel stay or car rental to cover themselves in case you run out of funds by the end. They may add in extras, such as room service fees, tips, taxes, even rental car damage fees, etc., that they estimate you will owe. The amount isn't actually gone from your account -- you just can't use it for other purchases.
It's not a problem for people who don't get near their limit. They probably aren't even aware that it has happened. But for people who live life close to the credit or debit edge, it could be trouble.