I just want to point out that for at least Lost and Found (finding Quina in the Witchwood), this quest has to be done before you do the quest in the Everfall for Barnaby in Gran Soren.
What triggers the failure is receiving your Wyrm Hunt License from the guard when you try to exit. It's exactly what just happened to me (so I started over). Got out of Everfall, turned in the quest, left. The guard gave me the license and then it said the quest was failed.
I'm going to assume you may want to treat any quests you get from Cassardis this way (minus notice board quests).