The giant toys in the movie, Tetsuo's mutation and Kaori's fate (among others).
After Tetsuo first escapes from the lab and is rescued from the Clowns by Kaneda and crew, he gets assaulted by horrific flashes of the future, including a nightmarish hallucination where he falls and hits the ground and spills his guts onto the ground and tries to scoop them back up.
There's the nightmare scene. After undergoing the experiment, Tetsuo is left to recuperate in what looks like a child's bedroom. He wakes up to see all the toys come together and form some kind of gigantic teddy bear with a monster-like right hand and a milk-spewing toy car that proceed to try to kill him.
And then there's the mutation scene, where Tetsuo loses control of his powers and transforms into agiant human amoeba that consumes everything it touches, and we also get to see poor Kaori get slowly crushed to death in a bloody pulp inside of Tetsuo. The anguished cries of Tetsuo and Kaneda accentuate to the terror. And the music does not help...
"Help me, Kaneda! Help, please!"
The official soundtrack for AKIRA is able to be frightening without words. Never try to listen to this at night with the lights off.