cast_sender.js error: Failed to load resource: net::ERR_FAILED in Chrome

Apparently YouTube constantly polls for Google Cast scripts even if the extension isn't installed.

From one commenter:

... it appears that Chrome attempts to get cast_sender.js on pages that have YouTube content. I'm guessing when Chrome sees media that it can stream it attempts to access the Chromecast extension. When the extension isn't present, the error is thrown.

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The only solution I've come across is to install the Google Cast extension, whether you need it or not. You may then hide the toolbar button.

For more information and updates, see this SO question. Here's the official issue.

I'm going to add to the answer given before.

It's not a bug in your code or the browser's code. It's the JavaScript code inside the YouTube iframe polls for the extensions it could interoperate with in case they were installed (likely to determine if the extension is installed).

Look at the source of www-embed-player.js (loaded from s.ytimg.com, it's YouTube static files CDN). You'll find the following:

function Wj(a){return"chrome-extension://"+a+"/cast_sender.js"}

To stop seeing those cast_sender.js errors, edit the youtube link in the iframe src and change embed to v