Field inspectors kept losing form submissions in basements and car parks. We rebuilt the app around a local-first write path: an IndexedDB outbox, an idempotent sync endpoint, version-based conflict detection and a UI that tells the truth about what is synced. Full working code included.
Order dates rendered fine locally but threw hydration errors for users in Asia and Europe. The server formatted dates in UTC and en-US, the browser in local time. Here is how we found it and fixed it for good.
After upgrading to Next.js 15 our dev console filled with sync dynamic API warnings, and one un-awaited auth helper quietly stopped redirecting logged-out users. How we migrated cookies(), headers() and params safely.