May's theme game - Unstable Magic
When you announce you are playing any spell at any time, you must roll 1 d12. If you roll a 1 through a 4, the spell 'fizzles' and goes back to your hand. The spell does not count as if it were played, but your mana remains tapped. If you are casting an instant and it fizzles, you may try and respond again, provided you have the free mana
You may pay 1 generic in addition to the casting cost of the spell so tha it only 'fizzles' on a 1 or 2, or two generic mana in addition to the casting cost of the spell so that it only fizzles on a 1. No amount of mana can be spent to guarantee a spell goes off.
Any spell cast via the phrase 'you may play this spell without paying its mana cost' are still held to the rules for fizzling. If the spell fizzles, it returns to the zone which it originally came from. In the result of cascade, the spell will be shuffled back into your library
Card restrictions - No Unglued/Unhinged
Deck Size limits - none