If you are running a mixed-mode 2.4/2.6 replica set and you execute an ensureIndex on a 2.4 primary with an index definition that does not match the existing definition, the 2.6.1-rc0 secondary will die with the following:
1. Create mixed version replica set with 2.4 as the primary.
2. On the primary
The 2.4 primary will ignore the second ensureIndex. However, the generated oplog will cause the secondary to crash with the above error.