The mongod process may crash if an index being used by an aggregation operation is dropped before the operation finishes.
The mongod process may crash in this scenario and needs to be restarted.
Do not drop indices that may be in use by aggregation operations.
MongoDB 2.6 production releases up to 2.6.4 are affected by this issue.
The fix is included in the 2.6.5 production release.
When dropping an index, correctly interrupt any aggregation operations that are using it.