As an example, I would assume worker is making requests with the internal view of the site, but can not have an internal view of other sites or security problems would ensue.. So what happens when two of my sites have service workers fetching something from each other on each request?
If a request bounces back such that the same worker script would need to run twice as a result of a single original request, then it fails with an error. There's nothing else we can do here: we can't let the request loop, but we also can't let it skip your script after it's bounced through a third-party script.
Does this hold true even for subrequests to your own zone, where you say here that they go directly to the origin server?