When an index build fails (for example, because the index is a "2dsphere" index and the collection contains invalid geometry), it can return the failure before the startIndexBuilds oplog entry is majority committed.

A stepdown at just the right moment can make it appear that the index is still being built (according to the listIndexes command), even though the client has already observed the failure code.

Waiting for majority commit in the failure case should stop this from happening.