Kinnan, Gwenna, and Foretell cards
Setup: Turn Player controls "Gwenna, Eyes of Gaea" that isn't affected by summoning sickness (i.e. can tap for mana as per its ability), and a "Kinnan, Bonder Prodigy". Player must have exactly one untapped mana source besides the Gwenna (in my case where I discovered this, it was a Basic Snow Forest), and they must have a card with Foretell in their hand (in my case, it was Ravenform). Bug was experienced on Windows 11 Steam version of Arena.
The Bug: The Arena client allows the Turn Player to pay the Foretell cost, despite the restriction on the mana Gwenna produces that it may only be spent " to cast creature spells or activate abilities of a creature or creature card," neither of which apply to foretelling Ravenform. I'm not a rules expert, so I'm unsure if Creature cards (like Augury Raven) should be allowed to have their Foretell costs paid in this setup, but even if so, the behavior with Ravenform should still constitute a bug, if I'm understanding all the cards involved here correctly.
Moistnar#18498
shared this idea
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Rezzahan#77802
commented
The key here is Kinnan, which creates one unrestricted use mana when activating Gwenna in addition to Gwenna's restricted use mana.
"Whenever you tap a nonland permanent for mana, add one mana of any type that permanent produced."
That mana comes from Kinnan and thus can be used on foretell costs.
Also, you cannot use Gwenna's mana to foretell a creature card. Foretell is a special action, so neither a creature spell, nor an activated ability of a creature or creature card.