lemmy does not return 502 error codes because 502 means "bad gateway" and lemmy is not acting as a gateway, nginx is. An nginx sends the html. All apps should check the status code for codes like this one that don't come from apps.

You in fact can yell fire in a theater without being arrested. It depends on the context (and weather or not there where injuries)

If the play calls for it and an actor says it. Or more simply if the theater is on fire.

With speech laws it matters more what the context is to the intended audience than what is specifically said.