As a fan, have you ever been seriously disappointed by an anime or manga series? Say for example you were a fan of a manga and they developed the anime for it. Your really looking forward to seeing it when it came out and when you finally did, you realized it didn't effectively capture the manga at all. Or even the other way around. You loved an anime series so you decided to check out the manga and much to your disappointment it sucked.
For me recently I would have to say that I was let down by the Wolf's Rain manga. Now I didn't think it was a
bad manga but it felt like you missed out on so much. They tried to cram 30 episodes of show into only 2 volumes. Now
I liked some of the things they added to the manga but unfortunately that meant that even more of the story had to be
cut out. So while the manga was not technically "bad" it just didn't hold a candle to the anime.
That's why to me it was a let down.