A site I developed was being critiqued by a fellow director. They looked at the HTML and didn't like the poorly written advertising and analytics Javascript near the start of it.
But wait! What advertising and analytics? I didn't add that sort of junk.
It took us a few rounds of me defending my design decisions and not understanding what their problem with it was, and them becoming suspicious of me, before we figured out they were looking at Javascript inserted by their ISP in real-time into the site's HTML. Not something I wrote. We were viewing different HTML because of that.
That was 6 years ago. One more reason to switch to HTTPS, even for public, static content.