@rhessinger Thank you!

I didn't realise support tickets were only for paying customers, even when reporting a bug. So anyway, here's my bug report again, requested hoop jumped through:

===What version does this affect?===
ProGet 5.3.12

===How can we help? (in brief)===
[Minor] The server should respond with an error for missing packages

===Please add any details that might help us help you.===
When searching for a package that is not part of a v3 feed, Proget responds with something like this:
{"count":1,"items":[{"count":0,"items":[],"parent":"http://private.server/nuget/feed/v3/registrations-gz/robblerobble/index.json","lower":null,"upper":null}]}
This response causes nuget.exe to crash and thus fails to install packages from other sources, regardless of the ordering of sources. The Nuget client team say the server should return a 404 error in this case instead.

There is additional discussion and a client log here:
https://github.com/NuGet/Home/issues/10107