So I was opening a pack of Emerald Dream (Android mobile, latest update) and somehow skipped over a card, which remained hidden and unrevealed and was unable to exit the pack opening screen. I shut down the app, restarted and everything was back to normal and the cards had been added to my Collection.
The funny thing? That hidden card I couldn’t see until I rebooted the app was a Signature Shaladrassil! I guess Hearthstone could not handle its power. That may be the culprit? It may also have conflicted with the Golden Mother Duck that was the cars right after.