When your code just doesn't work without any apparent reason, and after hours you realise that it's due to a misplaced }. Semicolons are overrated.

@Artemix It was JavaScript. This happened a couple days ago so I don't remember the details, but the } was misplaced in a way that code remained syntactically correct, but the it got split up in a way that nothing meaningful happened when I ran the function. So I put dozens of console.logs in the code to see at what point it stops running, and I found the problem. But the first few I hours I thought something went wrong on the server, because I didn't get any JS errors.

Or if you put == instead of !=