I encountered the "your ISP injects broken Javascript into all HTTP pages" problem around 2016. Someone thought I had done a poor job on a site I built for the

I encountered the "your ISP injects broken Javascript into all HTTP pages" problem around 2016. Someone thought I had done a poor job on a site I built for them. It turned out the Javascript on the page wasn't from the site I built, it was injected by the ISP, one of the UK's largest mobile data providers. We fixed the problem by switching to HTTPS.