Continuity mistake: When he's flying into the lake after hitting the bird, he goes from just about landing on the beach to landing in the very middle of the lake. No way he would have made it that far out, compared to where he was headed in the last shot. You can see how shallow the water is, and then it cuts to extremely deep water. (00:56:40)
Continuity mistake: When he has the car accident, the blood splat from hitting the bird goes from being an eight inch wide bright red smear of just "blood" that is low and centered directly in front of the driver to a four inch circle with chunks that is high and to the left. It is also totally missing while the car is viewed from above before hitting the water. (01:01:15)
Answer: This film is mostly open up for interpretation but I understand it that young Nemo thought up older Nemo and it actually happened, maybe in an alternate universe. Don't forget that Nemo could remember the future so most of what he imagines is probably going to happen. Unfortunately he can't predict choice so he doesn't know which life happens so all of them happen at once and they don't happen as well because he chose to make no choice (Zugzwang). Hope that explained it.