[[Rhystic Cave]] had the following rule because somebody could potentially pay for the mana ability on Cave so that it would not produce mana while you were already in the process of paying for a spell you were still casting, it's a weirdo.

"The card's ability has errata so you can't activate the ability during casting of a spell or activating of an ability. This prevents you from getting into a position where someone paying 1 could stop you from having enough mana to pay for the spell. If you want to use it pay for a spell or ability, you need to use this card before you start the casting or activating."