Otherwise waiters on the drop will proceed before the drop finishes. Waiters call getEarliestDropOpTime() to determine until what optime they should wait, and getEarliestDropOpTime() looks at _dropPendingNamespaces to determine that.

getEarliestDropOpTime() can currently see that _dropPendingNamespaces is empty, when there are still pending-drops to be completed, and return boost::none – or some misinformed answer --, which tells the caller that there's nothing for which to wait.

Discovered while testing SERVER-33244's patch on no_implicit_collection_creation_in_txn.js