Currently a filler collection is created in order to create the db in case it doesn't already exist https://github.com/10gen/mongo/blob/97cdfef9e4f74918176f1916bc53ada8f9f66868/jstests/core/explain_missing_collection.js#L8. However, explain_missing_collection might run twice on the same cluster, causing createCollection to fail. A check to prevent that should be implemented