In beta visit a page with nearby pages and click an item.
Click the read in full page and you'll be taken to an invalid URL e.g. http://localhost.beta/w/index.php/%3Cspan%3EMohawk%2C_Herkimer_County%2C_New_York%3C/span%3E
Example page where you can replicate this:
https://en.m.wikipedia.org/wiki/New%20York?mobileaction=beta#preview/geonotahack/43.0103,-75.005/Mohawk, Herkimer County,