9. Fruits Basket (2001)
When adapting another's work to the screen for the first time, it probably helps significantly if the director you hire gets along with the author of the adapted work. A lesson Studio Deen could have taken more to heart in 2001 when they hired Akitaro Daichi to adapt the revolutionary shoujo manga Fruits Basket for the first time.
Daichi and mangaka Natsuki Takaya HATED working together, reportedly butting heads constantly during the anime's production over very different creative directions they wanted to take the series. Of course, this heavily affected how the series adapted its source material.
Besides the manga being nowhere close to being done - so details like the true villain of the piece being a LOT closer to home than the manga's original readership anticipated never made it to the anime version - Daichi simply had different ideas on where to take the show, especially since they never got a second season.
Up to there, the fandom tolerated the ending to the original anime because hey, the rest of the anime is still good, so what if the ending makes LITERALLY no sense with the magic system at play in Fruits Basket? But now that we have the new anime that IS going to adapt the entire story, the original anime is a lot less forgivable.