In InitialSyncTest::playResponses, it's possible for the next ready request to be canceled by DataReplicator::_cancelAllHandles_inlock() after getNextReadyRequest() returns true.

https://github.com/mongodb/mongo/blob/965066ff4441f95ca757a6a9845fc1b70213e11e/src/mongo/db/repl/data_replicator_test.cpp#L656

                auto noi = net->getNextReadyRequest();
                if (isOplogGetMore(noi)) {