Cards that are banned in a specific event should be clearly marked during that event to prevent players from accidentally crafting them. Alternatively, allow players to disenchant recently crafted cards within a limited timeframe — say, 30 minutes or an hour. This would greatly reduce frustration caused by accidental or useless crafting.
For example, I crafted the quest “Escape the Underfel” specifically to use in this week’s Brawliseum, only to find out it was banned for the event. I felt somewhat tricked into spending dust on a card I couldn’t even use.
At the very least, support staff should be empowered to refund dust for recently crafted cards in justified exceptional cases.