So I just completed the amateur and expert mycologist quests and went to Faustus to collect my reward.
He offered me a potion. I drank it.
It gave me

warning - minor spoiler ahead!
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..
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huge ears, but nothing else, as far as I can tell. Talking to him again got rid of the big ears
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Completing the quest also gave me 100xp, still I wonder - does the potion have any additional effect I'm not aware of?

Yeah, talk about a disappointment. I spent so much time looking for these mushrooms...

You kidding, best reward ever xD
Burst out laughing when I got those ears and kept them all along just to read the other NPC comments ^^
So funny :))