My deployed site works locally but breaks in production — help?

This is one of the most common beginner traps, so let's make a checklist others can reuse.

Usual suspects when "it works on my machine" but not in production:

• Environment variables missing on the host (API keys, database URLs).
• Case-sensitive file paths — Windows ignores case, Linux servers do not. Header.jsx ≠ header.jsx.
• Build vs. dev differences — run the production build locally first (npm run build).
• Hard-coded localhost URLs that don't exist on the server.

What's your exact error message and host (Vercel, Netlify, a VPS)? Paste it below and we'll dig in.