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: