Reopening closed tab causes JSON data to be displayed

Steps to reproduce

1. Open an issue in a new tab so that it has no back history (e.g. middle click #19245 (closed)).
2. Close issue tab.
3. Reopen issue tab (e.g. with control-shift-Ton Windows)

What is expected

1. The issue page is displayed.

What actually happens

JSON is returned and displayed in the browser (presumably for the JS to handle as an AJAX response)

This was tested in Chrome and Firefox. I do remember it occasionally happening with MR's, but I am unable to reproduce it.