Consider this: Why would we treat two spell cards any differently than announcing a spell and then laying down a land card?
If you want to get super technical, the player has violated the steps of casting a spell by announcing a spell but failing to put it onto the stack - but I don't think we have to get that technical even, as the announcement alone should be sufficient to tell us what was intended and should be happening. The physical cardboard is
relevant, but not strictly
important to that aspect of the game, in my opinion.
Fixing it and moving on is completely reasonable.
As to the specifics of what happens if we've progressed in the game before the issue is noticed, I don't think it's actually much more complicated. Investigate; do you think they cheated hoping to lucksack the card they needed to make the move seem legal? Whether yes or no, you've basically solved the problem in its entirety at that moment - the only thing that differs is whether you're fixing the game or asking the players to write statements.
Edited Rebecca Lawrence (July 24, 2016 12:36:05 PM)