As evidenced by this story about an Amex gift card purchased at a Duane Reade pharmacy but not actually activated, mistakes like this can happen.

It is a very smart idea to check the balance of your gift card as soon as possible after you purchase it to make sure it was activated correctly and the balance is as it should be.  It is also a good idea to keep your gift card receipt so you can prove your purchase should you have a problem later on.  When giving gift cards, include the receipt.  Some gift cards will require a receipt for replacement if it is lost of stolen.