The moment you realize that you have successfully beaten reality with your unit-tests...

There are unit-tests for ...
... the api returning a 408 Http StatusCode when an internal request times out.
... the react app take this status-code and fires an action to display a specific error message for the user.

Every bit of code runs just fine.

Deploy this hell of an app on the server. Dandy Doodle.

Do a smoketest of the new feature.
FAIL!
Chrome starts to crumble during runtime. The api Request freezes.
Firefox takes the 408 api response but fails to interpret it in react app.

So I began to wonder, what the hell is going on.

Actually I recognized that I had the glorious idea to return a clientside error code in a serverside api response.
Glorious stupidity :/

Finally I fixed the whole thingy by returning an 504 (Gateway timeout) instead of 408 (Clientside timeout)

Cheers!