Ah, good times.... we used to do similar things across the HP Apollo workstations at the place where a friend of mine worked (and I unofficially "borrowed" computing facilities for a project of my own -- though I did also contribute a substantial speed optimization to their main product, so nobody seemed to mind).

For april fools' day, on my first job as a troublesome ~16-year-old administering our university lab's firewall (academia was a different land when it comes to trust), I set up some automatic network-wide substring replacement filters for incoming HTTP responses to replace `<body` with something like `<body style="transform: rotate(0.1deg);"`. This was in the time before HTTPS was ubiquitous, so it worked on most websites.

Unfortunately, it broke some pages that lab users needed for school. I later learned one colleague wasn't able to complete a homework assignment because of my prank.

The joy of running Upside-Down-Ternet as transparent proxy on April Fool's day.

I last did this in about 2011-2012 and was shocked and/or pleased to discover many images on my friend's Xbox 360 dashboard turning upside down!!