The new card Yusri, Fortune's Flame has the clause, that if you win 5 flips off it's attack trigger ability, you can cast any card in your hand, for free!.
Whenever Yusri, Fortune’s Flame attacks, choose a number between 1 and 5. Flip that many coins. For each flip you win, draw a card. For each flip you lose, Yursi deals 2 damage to you. If you won five flips this way, you may cast spells from your hand this turn without paying their mana costs.
Normally this gives you a 1/32 chance (1 in 2 to the power of 5), but what happens if you have Krark's Thumb out?