The mongod instance (with WiredTiger storage engine) is down when there is insufficient disk space for the data volume. After adding more space to the dbpath and restart the mongod instance, count() returns incorrect value as below:

> db.users.count()
1527296
> db.users.find().itcount()
1605004

SERVER-19472 and DOCS-5682 mentioned that count may off by up to 1000 documents after an unclean shutdown. However, count off to 70k+ documents in my test.

Have tested 3.0.12, 3.2.6 and 3.3.6, all have the same issue.

db.collection.validate(true) will reset the value for count() to the correct number.