The network bugs that eat a whole day tend to have the same shape: the error points at the wrong layer.

A 407 that looks like the site blocking you, when it's the proxy refusing a password with an unencoded @ in it. A scraper that works on a laptop and times out on a server because the server's DNS resolver is somewhere else entirely. A curl call that exits 0 while the request never reached the site. A redirect loop that only Chrome sees, because of an HTTPS record nobody knew was published.

So we're curious what yours was. What's the network or proxy error that sent you looking in completely the wrong place, and what was it actually?

Bonus points for the one-line fix at the end. We'll read every answer and reply to each one.