Hello guys,
Recently, I received the requested debt validation from a collection agency. But I observed that the offer was accepted by me, in bold print on the card, states "Offer Expires on Oct 15, 2006". Since the offer was clearly expired, do I have remedy against the collection agency?

No way. That offer expiration date has absolutely no bearing on your responsibility for the debt. More important for you is the date the last payment was made. That determines the start of the statute of limitations (SOL).