Galazeth Prismari, Goldspan Dragon, Animating Faerie auto-pay interaction
With Galazeth and Goldspan Dragon in play, along with exactly two untapped Treasures (and no other mana sources) I played Bring to Life (Animating Faerie's adventure spell) to bring one of the treasure tokens to life. I intended to tap one treasure token with the ability granted by Galazeth and sacrifice the other with the ability granted by Goldspan Dragon to pay for the spell. Obviously I wanted to target the treasure that I was tapping and not sacrificing, however Arena did not make it clear which one would be sacrificed to generate the mana. Normally Arena does a good job of not doing stupid things, so I targeted one and assumed it would know not to sacrifice that one. Unfortunately, it sacrificed the treasure that I had targeted. Seems like the sort of thing you'd want to know so you can fix the AI, or at least make much more clear which one will be sacrificed.
As a workaround, I will have to sacrifice one manually before casting the spell to avoid this.