If you read from a secondary through a mongos, and the underlying replica set has a fail-over, such that the secondary becomes a primary, mongos will still keep reading from that node, even though it is now in violation of the read preference. (Connections are pinned.)

This behavior is fixed in 2.6, but not in the 2.4.x branch. It is suggested to backport the fix, or create a different fix in 2.4 such that read preferences are honored even in the face of replica fail-overs.