FastAPI route order impacts matching. A less specific route (e.g., `/posts/{id}`) placed before a more specific one (e.g., `/posts/latest`) causes the less specific route to incorrectly match requests intended for the more specific one, resulting in errors. Reordering routes or adjusting URLs resolves this. FastAPI processes routes top-down, finding the first match. This segment demonstrates a crucial aspect of FastAPI: route order significantly impacts request handling. The presenter creates a seemingly innocuous route (`/posts/latest`) that conflicts with an existing route (`/posts/{id}`) due to its position in the route definition. A request to `/posts/latest` unexpectedly triggers the `/posts/{id}` route, leading to a validation error because "latest" isn't an integer. This highlights the importance of carefully ordering routes in FastAPI to avoid unintended matches and unexpected behavior, especially when dealing with path parameters. The example showcases how FastAPI processes requests top-down, matching the first route that satisfies the request, even if a more specific route exists further down.