I called the hotel directly to ask what "proof" they would need to qualify for the rate, as I know I qualify. She looked up the rate and asked "someone" and told me on the phone it was from last year. It jogged my memory and I vaguely remembered the promo from last year.

So if I book, I know ahead of time (consciously and morally) it's an expired code.

I do have other locations booked just in case. It's not the best hotel in the area, but it's a dirt cheap price! Yes, far less than 50%.

I haven't broken rules and ethics before, but I do believe it's up to the hotel to double check things. Not that I have, but for instance, if you have a bonus buck which clearly states 2 night stay, but you book and only stay one and they accept the bonus buck and you never hear about it - isn't it the hotels wrong and not yours?