If push comes to shove, Marriott could correct this mistake by just removing your FNC from your account and dumping the points pulled for your stay. And then they could deal with the hotel in the background (if they needed to.) I'd say up until the SPG merger, they would have done that. Today though given experiences/posts we've seen in other threads, I'm not so sure they will any longer. Way back in the day (i.e. I'm old now), back when Marriott sent actual paper certs, I accidentally handed the front desk the wrong cert for that stay. Once I figured it out, I had to send in the actual cert and they sent me back the one I'd accidentally used. But back then benefits and how Marriott dealt their elites was different than today.