Set up two Nexus Repo 3.19.1 instances.
In the first one, enable firewall with quarantine against the "maven-central" proxy. Create a policy in IQ Server that will fail for every component downloaded, such as component age must be less than one day.
In the second 3.19.1 instance create a proxy repository pointing to the "maven-public" group repository of the first one.
Fire off 20 requests for components that are in Central to the proxy repository. Then fire one more, and you will see:
2019-11-04 14:11:03,275-0600 WARN [qtp65580899-152] *UNKNOWN org.sonatype.nexus.repository.maven.internal.proxy.MavenProxyFacet - Exception org.apache.http.conn.ConnectionPoolTimeoutException: Timeout waiting for connection from pool checking remote for update, proxy repo aaa failed to fetch org/apache/zookeeper/zookeeper/3.4.5/zookeeper-3.4.5.jar, content not in cache.
At this point the connection pool is exhausted, and the only way to recover is to restart Nexus Repo.