Sasameki Koto. I watched the anime before lectura the manga (since I didn't even know there was a manga until afterward which usually happens) and the ending of the anime was so stupid since it stopped at the 12th chapter of the manga when there's 53 chapters. Pissed me off so much >.>
For me that was Elfen Lied's ending they left so many preguntas and probably expected everyone to live happily ever after o something like that..so many preguntas were left unanswered..same for Furuba's anime it didn't really mostrar much compared to the manga it didn't dig all that deep into Akito-san's past and portrayed her character differently..and again so many preguntas were left unanswered.
durarara!! When r they gonna give Celty's head back o when is she gonna get it back after looking 4 it all this time !!! and the very ending nvr resolved that and thats what she had been looking 4 since the beginning and yet everyone still lives happily ever after. (her head's still sitting/floating in a small big container behind some bookshelf kept away from her! And 4 no freakin good reason!)