More like "We both broke this rule that said we had to do something. What should we do about it?"
It should be handled roughly the same way as someone missing bleed triggers on a figure, shooting and ignoring range, or moving a unit while stunned only to realize later.
In the nationals, the champion took a shot that was illegal, and they played a few more activations. Later, they realized it, and rewound the game. Because that's what you should do when someone breaks a required rule.