Skyseer's Chariot Bug
It appears Skyseer's Chariot is broken in Arena. My opponent named '+2 Mace' with the Chariot on the stack. The issue is that the Chariot then received a +2/2 bonus to power and toughness, just like it was equipped with the '+2 Mace' Equipment card.
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Rezzahan#77802 commented
Skyseer's Chariot is a bse 3/3, so it only got a +1/+1 bonus from somewhere, most likely another card on the field.
Edit: Yep, there's a Cloudspire Captain giving it +1/+1. So no Mace bonus anywhere.