From the help on mongoexport:

  -k [ --slaveOk ] arg (=1)             use secondaries for export if
                                        available, default true

On a sharded cluster this can lead to tragedy:

• It would be possible for the same document to appear twice (repeated _id) or even many times (orphans) which could break a subsequent import.

• Some documents can be missing entirely, even ones that have been present for years and never modified (an in-progress migration can cause this).

The default should not allow reading from secondaries. If we are concerned about a change in behavior without notice then make the -k parameter required when connected to a mongos (as opposed to optional).

It is better to fail totally and noisily than to fail partially but silently.