I've gotten this weird error a few times now. I thought at first it was just because I was clicking quickly, but I clicked as slow as possible and it still happened. First, I would get the You don't have all the items for this quest! message, despite having bought them. Then I click return to last page, and get the Oh no! You aren't on a quest! message. I do get paid for the quest, however I don't receive any items when this happens. Happens with anyone, but just occurred with the Wizard three times and Saggitarius within the past 10 minutes.

Just got this too with Quentin ://

You still get the items and sp, it happens when you click twice on the "finish quest"..