Javadoc generation in particular relies on fetching data from linked javadoc, such as Lucene's, in order to generate proper links. See https://maven.apache.org/plugins/maven-javadoc-plugin/javadoc-mojo.html#links
When Lucene's website is down (happened today for all of Apache's websites), or has a transient failure (happens from time to time, say every month or so), we get this warning:
And since we fail on Javadoc warnings, the whole build fails:
Failing on javadoc warnings makes sense: it means we won't generate invalid javadoc, or at least it makes it less likely. What makes less sense is relying on external infrastructure (other than our CI and Maven Central) during our build.
Ideally we would generate links without actually accessing the website.