Given the following scenario:

1. A document update begins for a {_id: 1234}
2. An _id entry is found for 1234 and the update yields to fetch the given document from disk
3. During the yield {_id: 1234} is deleted by another operation
4. The update operation fails as RUNNER_DEAD, returning an error

The update should probably return successful with nupdated: 0 rather than error out. Looks like this is an issue for 2.6 only.

https://github.com/mongodb/mongo/blob/v2.6/src/mongo/db/query/idhack_runner.cpp#L234-L236