When a strong story beginning devolves into a weak middle and a nonsensical end. Yes, I've seen stories like that. And I can think of a series or two that suffered the same problem (fanfiction or otherwise). My only published fanfic had (note past tense) a sequel which I have since disowned because it was so... just, sucky and lame. I didn't feel too bad about it when I wrote it but then I came back later and realized it was just nasty. Sometimes you have to wonder if the author wasn't replaced with some kind of doppelganger who was able to get their narrator's voice down but lacked their actual writing skills.