The engineering questions you get asked, and the bugs you actually hit. Drawn, one mechanism at a time, in the order the videos ship.
401 means 'I don't know who you are (authenticate yourself)'. 403 means 'I know who you are, and you don't have permission'. The naming mistake in HTTP and how to use them.
Your server returned 200 OK, Postman works fine, and your frontend console is red. CORS is not a server error; it is a browser security sandbox.
You changed the price an hour ago and somebody just paid the old one. HTTP gives a server exactly one way to remove something from a cache, and it is a side effect rather than a command.
The page said welcome back, with somebody else's name on it. Four questions decide whether something in the path may answer instead of your server — and the default, when you say nothing at all, is not 'don't cache'.
Every request came back 200 and nobody could buy anything. The last two digits have no categorisation role at all — and an error inside a success is a lie three separate systems believe.
REST is six constraints in one chapter of one dissertation, not a synonym for JSON over HTTP — and the spec's own example of getting it wrong is a link a crawler follows to delete your rows.
A CDN is not a faster server. It's a copy of your response nearer the person asking — and everything that surprises you about it follows from what HTTP lets a shared cache keep.
Three messages before a byte moves, an ACK that isn't a receipt, and the half a ninety-second video has to cut: how fast to send, how much to hold, and where QUIC takes the whole job away.
Four servers, three of which have never heard of the name you asked for, and a cache so you never do it twice — the referral chain in real dig output, plus the CNAME rule that breaks your apex.
Five negotiations run between Enter and the first pixel, and the one nearly every answer skips is that an address and a connection are different things.
