When specifying a valid GH credential to access a public GH repo (e.g. in jenkinsci), it tests as successful:

Credentials ok. Connected to https://github.com/jenkinsci/matrix-auth-plugin.

But when running the pipeline with the default branch discovery (discover PRs, trust users with Admin/Write access), exceptions will occur during the scan that the /collaborators URL is inaccessible.

java.io.FileNotFoundException: https://api.github.com/repos/jenkinsci/matrix-auth-plugin/collaborators/daniel-beck/permission 

I'd expect a warning in the credential validation test. While a pipeline can be configured to not need permission lookup, given it's both common and the default, that seems like a bug.