[Snake]

Shot in the Dark
In this case, the quest goes from (completed) to (Incomplete) due to Cat Detective removing the Quest items from you.
(I think? Because before turning in the quest, you get a (completed) ).
But, it still appears on the (✓) Finished Quests tab. So it's not a big deal, unlike the next one below this.



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Lock and Key
In this other case, the quest has been completed, but not moved to the (✓) Finished Quests tab.

[Snake]

Relogging fixes it, apparently.