I was doing some quests on Sorrowdusk Isle and happened to open my Adventure Compendium... all of my entries are completely out of whack. Most of my favor, explorable area, and quest entries are completely different than what they're supposed to be; in most cases the values are lower than they're supposed to be but in a few cases they're higher (saying I've beaten a quest on Hard when I haven't even beaten it on Normal, for instance).
This is the first time this has happened to me, and logging out and back in fixed it. My guess would be that the game was reading the data for someone else's character, but that's just an uneducated guess.
This didn't seem to affect my actual quests or quest log in any way, other than Slayer/Explorer/Rare Encounter quests that I've completed are not flagged as such. The quest descriptions for those quests still display the correct level of completion.
EDIT: Added results from logging out and back in
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This is an unrelated bug since it's been happening ever since I've started playing, but I may as well mention it... when I first log in, my Adventure Compendium has about a 50% chance of being completely blank when I open it. When this happens, the only way to un-blank it is to log out and back in.