When a write concern is provided to the applyOps command, we normally wait on the OpTime of whichever operation successfully completed last. This is erroneous, however, if the last operation in the array happens to be a write no-op and thus isn’t assigned an OpTime. Let the second to last operation in the applyOps be write A, the last operation in applyOps be write B. Let B do a no-op write and let the operation that caused B to be a no-op be C. If C has an OpTime ahead of A, then we won’t wait for C to be replicated and it could be rolled back, even though B was acknowledged. To fix this, we should wait for replication of the node’s last applied OpTime if the last write operation was a no-op write.