replset6.js tests the replication of renameCollection commands across databases i.e.

assert.commandWorked(admin.runCommand({
    renameCollection: "jstests_replsets_replset6_first.one",
    to: "jstests_replsets_replset6_second.two"
}));

renameCollection commands across databases are not atomic, and are comprised of several, individual oplog entries. The last oplog entry that gets logged is the drop of the source collection, and before that we rename the temp collection into its final destination. If there is a delay between the application of these ops, we might see the rename but not wait for the source collection to be dropped on secondary.