In order to support readConcern, 3.2 nodes log an n-op to the oplog, effectively creating a floor for what they will consider committed. 3.0 nodes ignore n-ops while processing the oplog. This can cause a problem wherein no node believes themselves to be electable:

• 3.2 low priority node is elected and logs an n-op
• that node steps down from primary due to a heartbeat from a higher priority node
• that higher priority node (3.0) will not run because it is not the freshest
• the 3.2 node will not run because it is not highest priority

Log containing example for mixed_storage_version_replication.js: https://logkeeper.mongodb.org/build/56295972be07c42d836c7ac3/test/5629599abe07c42d836c7ec7?raw=1