A 2.4.9 primary node executed ensureIndex for a preexisting named index, but using different keys than the existing index. (I.e. application code and reality were out of sync.) The 2.4.9 primary will simply ignore the ensureIndex. However, this causes a fatal replication error on the 2.6.0 secondary.
Note that you will only encounter this situation if all of the following are true:
- You have a 2.4.9 primary and a 2.6.0 secondary
- You are using named indexes
- You call ensureIndex with a spec that does not match the actual spec for the existing named index.
Here is a simple reproduction case with a 2.4.6 primary and a 2.6.0rc3 secondary:
Note that this issue does not affect 2.4.10; you must use 2.4.9 or older in order to be affected.