Cosmium blast
Playing LCI limited and trying to play cosmium blast with plenty of available mana and legitimate targets, but it wouldn’t let me which probably cost me the game.

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BlueMardler#64923 commented
Lost a draft game due to this bug which meant I missed out on gems and cards
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DAC169#34537 commented
Define "not working." Was it not letting you cast it? If so then your opponent likely had Kutzil, Malamet Exemplar out if you were in Draft, or Myrel, Shield of Argive if not. Or a Bedrock Tortoise if you couldn't target their creatures.
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Wiggidy_Wizard#89441 commented
Cosmium blast not working when opponents attack me... had two in hand and it most likely cost me the game after multiple combat phases
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[Deleted User] commented
Try reading your opponent's cards. They have Kutzil, Malamet Exemplar on the battlefield. It has the ability "your opponents can’t cast spells during your turn."