What you need to know
A project may work locally but fail in preview when a required secret or public URL is missing. Environment variables should be scoped to the environment that uses them.
How to apply it
Document the variable names and purpose, add local examples with blank placeholders, set values in the host dashboard, and redeploy after changes when required. Use public prefixes only for values safe to expose.
Common mistake to avoid
Do not share secret values in an issue, commit, or build log. Rotate any credential that was exposed.