MongoDB queries don’t always return all matching documents!
David Glasser
33136

Here is how you can easily reproduce the issue on MongoDB. I used about 300K document to force the issue to reproduce quite consistently. http://blog.couchbase.com/2016/july/testing-mongodb-and-couchbase-concurrent-query

This article also contains the comparison of the same test on Couchbase and I work for Couchbase.