SOLR-11218 Erick Erickson ensured that we can't delete a collection that is referenced by an alias. However It may be that the alias is deleted but the node servicing the request doesn't know about this yet. It should call AliasesManager.update() first (which now sync()'s with ZK).
I believe this is the cause of some sporadic failures to org.apache.solr.cloud.AliasIntegrationTest#tearDown which deletes the alias then all collections.
It's debatable if this is an improvement or a bug. Sadly most of SolrCloud simply seems to operate this way despite it being eventually consistent. Thus users using SolrCloud may have to add sleep()s after calls to Solr admin calls :-/