We have created a temporary collection for the purposes of map-reduce output. After running several map-reduce jobs that failed in the last moment due to a networking error, we wanted to inspect the stats on those collections.

About 30-50% of the collections fail on the "stats()" call but can be successfully iterated using "find()". I have also confirmed that the collections actually exist on every shard (by connecting to each shard's mongo and doing a stats() call there).

mongos> db.collection_nw1q.stats()
{
	"sharded" : true,
	"ok" : 0,
	"errmsg" : "failed on shard: { ok: 0.0, errmsg: \"ns not found\" }"
}
mongos> db.collection_nw1q.find({}, {_id: 1}).sort({_id: 1}).limit(20)
... actual data returned here, no error ...

There are no errors in mongos, even at logLevel: 1, when the failed stats() call returns.