Some incompatibility observed include:
1. Using v3.2 shell on v3.0 mongos can cause the mongos to ignore the read preference setting. This is because the read preference setting used to be inside the explain subdocument in v3.0 and is now moved to the top level in v3.2.
2. Using v3.0 shell and v3.0 mongos with v3.2 shards to run an explain with slaveOk will result in: