Cd to any Maven project that's lying around, then do this:
This fails, as expected, because the maven-clean-plugin doesn't exist locally. Now try this:
Even though now Maven is online, it still fails with the same error. No download is attempted (unless I add -U.)
This is because even in the offline mode, Maven creates ~/.m2/repository/org/apache/maven/plugins/maven-clean-plugin and put empty repository metadata. This causes the later online Maven build to believe that the repository was checked and no plugin was found there.
Maven shouldn't create an empty repository metadata in the local repository if In the offline mode.