You suddenly realize that for the past 4 turns you’ve been using a Power Card on lands it can’t actually target. What to do?

The answer is: don’t sweat it. Especially on your first play or two, you’ll likely make minor mistakes here and there. They’re unlikely to break anything: the game will just be slightly easier/harder.

So long as everyone’s having fun, it’s all good, there’s no need to try and “rewind” and fix things. Just take note of what the correct rule is, and use it going forward.