Our React app called https://undefined/v1/... in production while dev worked fine. Vite replaces import.meta.env at build time, and our Docker image set the variable only at runtime. We fixed the build and then moved to runtime config.
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.
Bumping React to 19 made npm install fail with ERESOLVE. The quick --legacy-peer-deps workaround installed, then crashed tooltips in staging. We learned to read the conflict, upgrade the real offender and use overrides only as a short, tested bridge.
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.
Our onboarding page created two draft workspaces per visit in development. StrictMode was blamed and nearly disabled, but the real bug was a non-idempotent POST inside an effect with no cleanup, and it was also happening in production.