In mixed_storage_version_replication.js, there are a bunch of threads that are dropping, creating, inserting, deleting, indexing, etc. To decide which collection to operate on, the test would listDatabases. If that database only had one collection, it would return null, otherwise while a random collection from the database was "system.indexes" it would re-list collections from that database. Every so often, it would hit a database that had two collections, and then while it was looking for a collection that wasn't "system.indexes", a collection would be dropped and then it would be in an infinite loop of looking for a new collection. This causes the test to hang.