Ok, so I know that my opponent doesn't need to pay 2 to attack my planeswalker if I have ghostly prison in play. http://www.mtgthesource.com/forums/s...ad.php?t=12819

But what if they don't realize that and pay 2 anyway? Can I be penalized for not informing my opponent that it isn't possible to pay for ghostly prison when they attack my planeswalker? Does this constitute "failure to maintain a legal gamestate" or just a situation where my opponent was supposed to know better? Does the REL or my knowledge of how the interaction is supposed to work change if I would get a penalty? Does it matter if my opponent specifically say "I'll pay 2 for the prison to attack Ajani?" as opposed to just tapping to lands and attacking?