The most frustrating error is logic error. You see no warning or error message. It's just your program doesn't work as expected.

You spend hours inspecting your code, putting var_dump, console.error, print, whatever... in order to figure out what's wrong.

Yet, you still can't find out what's wrong. You decide to take a sleep. And you are waking up while having this uncomfortable feeling in your mind.

And since this is devrant, I'll insert some swearing here:

F*cking dammit node.js... what's wrong with your f*cking logic flow.... non-blocking you said, rotten deep in callback hell damnation you said....