This was a way, dare I say, cooler bug involving closures around getGetServerSideProps parameters.

I thought I was being efficient by reusing them for storage inside the closure, but I was "enclosing" the changes to the parameters. The next time the function was called, it was using the enclosed values and not the parameters.

It's probably cache related but it could also be query params getting sent as null or something because it's the same handful of wrong users.