Durarara!! - Episode 9

Fans have noted that in the light novel, Celty's head is described as having hair "as black as darkness," whereas the anime depicts it with brown hair.

Celty finally spots the girl she believes has her head and pursues her. Shizuo intervenes to help, leading to a clash with Seiji, who stabs Shizuo with pens before being easily knocked unconscious. Durarara!! Episode 9

If you are watching the later seasons, here is a quick guide to their respective 9th episodes: Fans have noted that in the light novel,

While the anime presents flashbacks from Namie's perspective, the original light novel focuses on Seiji's perspective during these scenes. If you are watching the later seasons, here

It is revealed that Mika Harima , who had been stalking Seiji, discovered the head in his apartment. In a fit of rage and obsession, Seiji killed her. Namie later had the head (supposedly) surgically attached to Mika’s body.