Can't redeem an Ingress Passcode

Question

I've gotten a Passcode for Ingress that I know I've never used before. However, when I attempt to add it I'm getting a message ALREADY_REDEEMED. What's going on?

Answer 1

Passcodes are no longer unlimited.

Sources:

• http://ingress.gproductions.no/i-am-sorry-to-say-that-the-secret-passcodes-are-no-longer-for-unlimited-use-as-they-have-been-shared/
• http://decodeingress.wordpress.com/2012/11/29/ingress-passcodes-change/
• Ingress support forum