One of the co-signers of the Agile Manifesto had previously stated that "The best way to get the right answer on the Internet is not to ask a question; it's to post the wrong answer." (https://en.wikipedia.org/w/index.php?title=Ward_Cunningham#L...) I'm convinced that the Agile Manifesto was an attempt to make an internet post of the most-wrong way to manage a software projects, in hopes someone would correct it with the right answer, but instead it was adopted as-is.
The mangling of JavaScript to fit through every hole seems to be the biggest mistake made in modern programming, and I'm not sure what even keeps it going aside from momentum. At first it regained ground because Flash was going EOL, but now?