500 Error When Navigating Back and Forward on Mobile

[zicklag]

Report from user:

Cant repro on desktop, but in Firefox mobile on iOS:

If I go to weird.one from some other site, click a person’s profile, hit back until no longer on weird.one, and hit forward again, I get a error 500 on weird

I can get it in safari mobile too

Note: The 500 error was the nicely-styled one, not a blank page 500, which might give a clue as to where it's being caused.