Don't ask how I came across this one, it's a strange one, to reproduce,
This only happens in Firefox as far as I can tell,
Open a topic via ajax from a category, close the tab with ctrl + w or with the X on the tab, press Ctrl Shift & T to reopen that tab. Hold the back button on the browser and click the first link that takes you back to the category where you came from.
You get this:
However the URL is that of thr category.
It actually looks quite nice, but should probably be an option if people ask, and not a bug. (Would tie in nicely with the related posts issue that's on Github)