Something similar happened in a T2 game I had with Gabe (I think it was that game?). If I remember the situation correctly, I did something like go in with AutO to D2, then exchanged with Jair to D2. Either Jair was negated, or the exchange was to deck when Naz or Ring was up, I can't remember, but it was a very illegal play.
When we realized what had happened, we both talked about how to fix the issue. I put back the second pair of cards, did a legal exchange, and then suggested that I shuffle the second pair of cards back in completely, since I saw them (sadly, Guardian was one, and I had thrown it out already too). He agreed, and we resolved it quickly.
When errors happen, the first solution should always be to admit the mistake and suggest a solution that everyone can agree to, with preference given to the person who didn't mess up. If an agreement can't be made (which should be rare), a judge can make the call, but I can't remember the last time that happened in one of my games.
