we are seeing mongos output about 300 errors/sec of the following:
Fri Oct 7 14:15:12 [WriteBackListener-10.x:27021] writeback failed because of stale config, retrying attempts: 58673730
Fri Oct 7 14:15:12 [WriteBackListener-10.x:27021] writeback failed because of stale config, retrying attempts: 58673731
...

the attempts appear to be going to a shard which previously had a similar problem of outputting a ton of errors of the following:
Sat Sep 24 14:44:24 [conn175294] Assertion: 13388:[discovery.items] shard version not ok in Client::Context: client in sharded mode, but doesn't have version set for this collection: discovery.items myVersion: 1|98

this is with mongos nightly as of oct 6.

we are also seeing sporadic errors of "db assertion failed" when running map reduce jobs, similar to SERVER-3081:
Wed Sep 28 03:27:08 [conn11550] ERROR: sharded m/r failed on shard:
shard1/10.x:27017,10.y:27017 error:

{ assertion: "[mydb.mycoll] shard version not ok in Client::Context: client in sharded mode, but doesn't have version set for this collection...", assertionCode: 13388, errmsg: "db assertion failure", ok: 0.0 }

Thu Sep 29 02:12:13 [conn52340] ERROR: sharded m/r failed on shard:
shard4/10.x:27017,10.y:27017 error:

{ assertion: "[mydb.mycoll] shard version not ok in Client::Context: client in sharded mode, but doesn't have version set for this collection...", assertionCode: 13388, errmsg: "db assertion failure", ok: 0.0 }

Wed Oct 5 14:50:03 [conn360741] ERROR: sharded m/r failed on shard:
shard1/10.x:27017,10.y:27017 error:

{ assertion: "[mydb.mycoll] shard version not ok in Client::Context: client in sharded mode, but doesn't have version set for this collection...", assertionCode: 13388, errmsg: "db assertion failure", ok: 0.0 }

Fri Oct 7 02:07:41 [conn26339] ERROR: sharded m/r failed on shard:
shard2/10.x:27017,10.y:27017 error:

{ assertion: "[mydb.mycoll] shard version not ok in Client::Context: client in sharded mode, but doesn't have version set for this collection: revinate_p...", assertionCode: 13388, errmsg: "db assertion failure", ok: 0.0 }