I think this is a good reproducer, but y'all can grab the stack trace out of your logs. I figured you should be aware, since the error might be self-explainable enough to figure out quickly.

While looking back at D17304, I tried clicking on 'Show Older Changes', and got this popup:

If I click it and make it go away, every time I try again it fails. But if I hit ctrl+r and reload, then it seems to work.

I actually just found out, this only seems to occur reliably if I visit D17304 from https://secure.phabricator.com/rPab923e0a752b9dc03a850be74d198832a5620373 -- so:

• Start off by visiting rPab923e0a752b9dc03a850be74d198832a5620373 (in a new tab)
• Scroll down below the summary, and click on the link to the revision in the metadata box. To be clear, click this link:
  
• When you land on the page for D17304, try clicking "Show Older Changes", and watch it explode.