Naturally this is a Brian Krebs post. Apparently this is well-known, but there is a technique where fraudsters are either able to lift off the "scratch-proof" seal on the back of the card that protects the activation code, or they replace it, and suck the value of the card. So you end up paying $25, or whatever, for a piece of plastic, and give something with $0 value.
The top two have been tampered with, the bottom one is intact:

https://krebsonsecurity.com/2017/12/buyers-beware-of-tampered-gift-cards
The top two have been tampered with, the bottom one is intact:

https://krebsonsecurity.com/2017/12/buyers-beware-of-tampered-gift-cards