I had on the battlefield Toski, Bearer of Secrets and due to is effect "Toski, Bearer of Secrets attacks each combat if able." and only 1 mana untap. My opponant had Ghostly Prison make my creatures can't attack him unless I pays {2} for each creature attacking him.

So with only 1 mana I can pay the Ghostly Prison tax for 0 attaking creatures, logicaly the clause "if able" make I will make Toski don't have to attack.

But the it was not the case, and I was trap with a Tosky need to attack (unability to not declare attack) and when I declare Toski attack force to cancel because I have not the mana for the tax.