This bug is about a node should be compressed witch is not.
The following code can create that node.

TEST("limit")
{
        quicklist *ql = quicklistNew(2, 1);
        quicklistPushHead(ql, "0", 1);
        quicklistPushHead(ql, "1", 1);
        quicklistPushHead(ql, "2", 1);
        quicklistPushHead(ql, "3", 1);
        quicklistPushHead(ql, "4", 1);
        quicklistPushHead(ql, "5", 1);

        assert(ql->len == 3);
        assert(ql->head->next->encoding == QUICKLIST_NODE_ENCODING_RAW);

        size_t sz = (1 << 12);
        unsigned char *s = zmalloc(sz);
        randstring(s, sz);
        quicklistEntry entry;
        quicklistIter *iter = quicklistGetIteratorEntryAtIdx(ql, 2, &entry);
        quicklistDelEntry(iter, &entry);
        quicklistInsertAfter(iter, &entry, s, sz);
        quicklistReleaseIterator(iter);
        assert(ql->len == 3);
        /* ql->head->next is not compressed */
        assert(ql->head->next->encoding == QUICKLIST_NODE_ENCODING_RAW);

        /* ql->head->next should be compressed */
        __quicklistCompress(ql, ql->head->next);
        assert(ql->head->next->encoding == QUICKLIST_NODE_ENCODING_LZF);

        zfree(s);
        quicklistRelease(ql);
}

This bug is related with the member recompress in struct quicklistNode.
A node will not be compressed if it's not compress small enough. And it will remain uncompressed after insert, because 'recompress' is 0 and will prevent compression。