Startup two 2.6 mongod's, and one 2.8 mongod, all the same replica set. Connect to one of the 2.6 nodes, and try to initialize the replica set. Bash history here: ➜ mongo git:(CAP-1250) ✗ ls /data/db (CAP-1250⚡) mixed_storage_and_version-0 mixed_storage_and_version-1 mixed_storage_and_version-2 mixed_storage_and_version-3 mixed_storage_and_version-4 mixed_storage_and_version-5 mixed_storage_and_version-6 sconsTests ➜ mongo git:(CAP-1250) ✗ mkdir /data/db/mixed_version (CAP-1250⚡) ➜ mongo git:(CAP-1250) ✗ mkdir /data/db/mixed_version/node0 (CAP-1250⚡) ➜ mongo git:(CAP-1250) ✗ mkdir /data/db/mixed_version/node1 (CAP-1250⚡) ➜ mongo git:(CAP-1250) ✗ mkdir /data/db/mixed_version/node2 (CAP-1250⚡) ➜ mongo git:(CAP-1250) ✗ ./mongod-2.6 --dbpath /data/db/mixed_version/node0 --logpath /data/db/mixed_version/node0/mongod.log --logappend --smallfiles --noprealloc --nopreallocj --replSet mixedVersion --fork (CAP-1250⚡) note: noprealloc may hurt performance in many applications about to fork child process, waiting until server is ready for connections. forked process: 12658 child process started successfully, parent exiting ➜ mongo git:(CAP-1250) ✗ ./mongod-2.6 --port 27018 --dbpath /data/db/mixed_version/node1 --logpath /data/db/mixed_version/node1/mongod.log --logappend --smallfiles --noprealloc --nopreallocj --replSet mixedVersion --fork (CAP-1250⚡) note: noprealloc may hurt performance in many applications about to fork child process, waiting until server is ready for connections. forked process: 12670 child process started successfully, parent exiting ➜ mongo git:(CAP-1250) ✗ ./mongod --port 27019 --dbpath /data/db/mixed_version/node2 --logpath /data/db/mixed_version/node2/mongod.log --logappend --smallfiles --noprealloc --nopreallocj --replSet mixedVersion --fork (CAP-1250⚡) note: noprealloc may hurt performance in many applications about to fork child process, waiting until server is ready for connections. forked process: 12682 child process started successfully, parent exiting ➜ mongo git:(CAP-1250) ✗ ./mongo --port 27017 (CAP-1250⚡) MongoDB shell version: 2.8.0-rc1-pre- connecting to: 127.0.0.1:27017/test > rs.initiate({_id: 'mixedVersion', version: 1, members: [{_id: 0, host: 'charlie-macbook-pro:27017'}, {_id: 1, host: 'charlie-macbook-pro:27018'}, {_id: 2, host: 'charlie-macbook-pro:27019'}]}) { "ok" : 0, "errmsg" : "couldn't initiate : member charlie-macbook-pro:27019 is already initiated" } ./mongo --port 27019 (CAP-1250⚡) MongoDB shell version: 2.8.0-rc1-pre- connecting to: 127.0.0.1:27019/test > rs.initiate({_id: 'mixedVersion', version: 1, members: [{_id: 0, host: 'charlie-macbook-pro:27017'}, {_id: 1, host: 'charlie-macbook-pro:27018'}, {_id: 2, host: 'charlie-macbook-pro:27019'}]}) { "ok" : 1 } mixedVersion:OTHER>