This happens often on AppVeyor and I've seen it locally occasionally too, but I also think I've seen it on Travis so it may not be Windows-specific. We get either status code 500, or "Failed to receive response: The server returned an invalid or unrecognized response," etc.

We have made recent changes in libgit2, adding a few new tests and making it throw a few more errors to avoid catching unintended things like typos. Whatever is happening here seems like it may be due to intermittent connectivity problems, or maybe an underlying bug/race condition/undefined behavior in the C library interfaces? Either way we should try to pinpoint the cause and add a mitigation (retries on some operations? specific handling of this expected failure mode?) or fix it if it's a real bug.

What's the best way to debug this? Does anyone else see this locally if you run make test-pkg in a loop? cc @wildart

edit: this looks a lot like #13436 but apparently that had been more common on 32-bit?