You meet Shaella in the "Blade and Stars Inn" first floor (southeast section of Baldur's Gate). She's a priest of Leira, the god of mist and the unknow. By always answer her with questions or statements that you don't understand what she wants, she will be most pleased and will grant you the "Find the Great Book of the Unknowing" quest.
"Find the Great Book of the Unknowing" consists in find the sacred book of Leira inside Candlekeep, it's a book that confers ignorance instead knowledge.
In the old game, this quest didn't made any journal entry, now it does. So many people on the old time thought at this quest as an easter egg and of D&D and a funny joke, as you complete the quest by not completing the quest (however you didn't get any reward or note on your journal as a done quest).
Now that the quest get an register on the player's journal, did the devs fixed the quest? It's possible now to complete it?
So to follow the rules:
Current Behavior -
Find the Great Book of the Unknowing quest is recorded on the journal but impossible to be concluded.
Expected Behavior -
Fix the quest and give the player a conclusion that will send the note on the journal to the done quests section.