Today I was playing a normal arena when I was against rogue at the start he was playing like a normal player but at turn 4 he started to play wild cards, for example sludge belcher. Sadly I dont have any video of it, because then my ethernet broke and I couldnt get back in the game again. I'm not sure about the player name but it was something similar to Thrgs.
Does exist a bug or hack that allows you to have wild cards in arena, while not playing the heroic tavern brawl?
He started his arena before february 2017 (that's when arena became standard, it had no formats before, all the cards were allowed). Probably forgot about it, or stopped playing and get back into the game just in time for next expansion