Error codes 403, 404 and 500: what do they mean?

Three numbers that tell you where to look. Confusing them wastes time.

403 — forbidden

The server finds the file but refuses to show it. Almost always a matter of file permissions or a rule in your configuration.

404 — not found

The address matches nothing. After a move or a URL change this points to links still aimed at the old structure. Redirect them rather than letting them die.

500 — server error

Something broke during processing. The number itself says nothing: the real information is in the error log, not on screen.

Which is urgent

A 500 is always urgent. A 403 usually is too. A 404 is annoying but rarely critical — unless it hits an important page.

What the visitor sees

A default error page drives people away. A page that explains and offers a way back keeps some of those visitors.