HTTP cookbook

You call an API, get nothing back that makes sense, and you can't tell if the problem is yours or theirs.

A response looks compressed. You've never checked.

A link redirects, then redirects again, and never lands anywhere.

You have an API token in your hand, but you've never proven that the API rejects a call without it and accepts one with it.

The status code says 200. The request failed.

Your fetch call throws a network error in the console, but the API itself works fine everywhere else you've tried it.

Why does your JSON endpoint sometimes answer back in HTML instead?

You log into a site with no idea whether the cookie it just set can be stolen by a script on the page.

You assume your browser sends clean headers. You've never checked the raw bytes.

Two people save the same record minutes apart, and whoever saves second wins, with no warning that the first change vanished.