The animation goof in the movie. The animation goof has Nausicaä's dress mysteriously change completely blue when another animation sequence is shown. However, during that split second, the Ohmu managed to bleed so much it dyed her dress entirely blue (When before it wasn't shown as instantly turning it blue) and even managed to get on her back. The Ohmu bled that much.
It's not an animation goof, it's deliberate. The baby Ohmu was deliberately shown spurting blood like a fire hose for precisely this reason.
In the live action short by Hideaki Anno, the Ohmu uses its bright purple laser cannon on the city of Tokyo, obliterating most of it in a few minutes. Flash forward four years, and Anno is working on a new film, with another giant monster who can use a purple beam that's similar to a laser. Who is this monster? It's Shin Godzilla!
The flashback of a younger Nausicaa wanting to keep a baby Ohmu as a pet, on first viewing, doesn't really seem to have much significance to the plot. However, if you think about it, Nausicaa's flashback - in which a baby Ohmu is depicted as harmless and helpless - encourages the audience to feel sympathy for a member of a species shown through earlier parts of the film to be dangerous. What happens in a later scene? A baby Ohmu is used as bait for the rest of its species. If the horrifyingway the Ohmu is used doesn't encourage the audience to feel bad for it, the earlier flashback scene - even on a subconscious level - does. In short, the film uses a seemingly random flashback to manipulate the emotions of the audience in preparation for the climax. Well played, Miyazaki-san.
Near the end of the movie, Nausicaa wears a red dress. Moments later, after stopping the stampeding Ohmu, her dress is blue. What else is red and blue? The Ohmu's eyes, of course! And the timing is just right, too.