If a mapreduce is interrupted by a stepdown while its "_inc" collection exists, it will not be able to delete the "_inc" collection. Since that collection is not replicated (per
SERVER-13981), this will result in a DBHash mismatch.
It might be better to come up with a better way of handling those collections, such as putting them in local.tmp. As it is I wonder if initial-syncing nodes will pick up the _inc collections even though they are not in the oplog.