For very expensive and long-running aggregation operations (query/m-r/agg) in very active clusters, the retry logic in parallel.cpp may need to be more aggressive. The default number of retries in this case may not be appropriate - explicit progress tracking is probably a better approach.

See intermittent error:
http://buildlogs.mongodb.org/MCI_linux-64/builds/289241/test/slow2_0/mr_during_migrate.js