Constant Deck Glow followed by uncastable Eerie Ultimatum in direct Challenge
Playing Brawl against a friend in Direct Challenge, at some point his deck started glowing and showing blue particle effects as if he was searching library or something. Effect lasted multiple turns through drawing and milling cards. Then, he cast Eerie Ultimatum from his hand, paying the full mana cost, but the spell fizzled. There was no Teferi, Containment Priest, or anything else that should have stopped the effect, but he was unable to view the cards in library, unable to select any, and the spell resolved without doing anything. Shortly after this misfire, the glowing deck effect disappeared. We played the rest of the game without any issues, but it was a pretty frustrating turn and nearly lost him the game!