Copying Fling does not work as it should
Reproduction steps:
- put Yargle, Glutton of Urborg onto the battlefield via DevMode
- put Refuse // Cooperate into your graveyard via DevMode
- put Fling into your hand via DevMode
- pass the turn
- during first main phase, Generate Mana via DevMode
- cast Fling from your hand
- select your opponent as target
- pay the mana cost
- pay the additional creature sacrifice by selecting Yargle, Glutton of Urborg
- while Fling is on the stack, cast Cooperate from your graveyard, targeting Fling
- target your opponent with new copy of Fling
- let the stack resolve
Opponent should take 18 damage, but only takes 9.
Judge Ruling: https://blogs.magicjudges.org/rulestips/2012/10/you-dont-have-to-sacrifice-a-creature-when-you-copy-fling/
Original report: https://www.slightlymagic.net/forum/viewtopic.php?f=26&t=22137&p=224521#p224520