Sorry, I lost track of the original post, which fits Toby's description of a simple backup; I was still thinking about the various rabbit holes added on, which would be more than just the last action and thus be addressed by the partial fix.

Isaac, you seem to be separating a single action (playing an ability) into sub-atomic particles (heh); the playing of the Eternalize ability includes a mana cost, a discard, and creating a token. In the original scenario, the player forgot one of the steps of that action; if it's pointed out right away that a discard is also required for that action, then a simple backup of the entire action is fine.

d:^D