For example, I was looking for user with _id : 35713909
mongos gave me nothing. When I queried the shard directly, I found the record.
Restarting mongod's solves the issue.

> db.printShardingStatus()
— Sharding Status —
sharding version:

{ "_id" : 1, "version" : 3 }

shards:

{ "_id" : "moscow", "host" : "moscow/moskau7:27018" } { "_id" : "brussels", "host" : "brussels/moskau8:27018" }

databases:

{ "_id" : "admin", "partitioned" : false, "primary" : "config" } { "_id" : "pravdorub_production", "partitioned" : true, "primary" : "brussels" }

pravdorub_production.vk_users chunks:
{ "_id" :

{ $minKey : 1 }

} -->>

{ "_id" : 1394216 }

on : moscow

{ "t" : 2000, "i" : 0 }

{ "_id" : 1394216 }

-->>

{ "_id" : 3034926 }

on : moscow

{ "t" : 3000, "i" : 0 }

{ "_id" : 3034926 }

-->>

{ "_id" : 4822146 }

on : moscow

{ "t" : 4000, "i" : 0 }

{ "_id" : 4822146 }

-->>

{ "_id" : 6717185 }

on : moscow

{ "t" : 5000, "i" : 0 }

{ "_id" : 6717185 }

-->>

{ "_id" : 8766778 }

on : moscow

{ "t" : 6000, "i" : 0 }

{ "_id" : 8766778 }

-->>

{ "_id" : 10955178 }

on : moscow

{ "t" : 7000, "i" : 0 }

{ "_id" : 10955178 }

-->>

{ "_id" : 13126517 }

on : moscow

{ "t" : 8000, "i" : 0 }

{ "_id" : 13126517 }

-->>

{ "_id" : 15627074 }

on : moscow

{ "t" : 9000, "i" : 0 }

{ "_id" : 15627074 }

-->>

{ "_id" : 17905562 }

on : moscow

{ "t" : 10000, "i" : 0 }

{ "_id" : 17905562 }

-->>

{ "_id" : 20203553 }

on : moscow

{ "t" : 11000, "i" : 0 }

{ "_id" : 20203553 }

-->>

{ "_id" : 22536604 }

on : moscow

{ "t" : 12000, "i" : 0 }

{ "_id" : 22536604 }

-->>

{ "_id" : 25386314 }

on : brussels

{ "t" : 1000, "i" : 12 }

{ "_id" : 25386314 }

-->>

{ "_id" : 28441578 }

on : brussels

{ "t" : 1000, "i" : 13 }

{ "_id" : 28441578 }

-->>

{ "_id" : 31399808 }

on : brussels

{ "t" : 1000, "i" : 14 }

{ "_id" : 31399808 }

-->>

{ "_id" : 34555646 }

on : brussels

{ "t" : 1000, "i" : 15 }

{ "_id" : 34555646 }

-->>

{ "_id" : 38531058 }

on : brussels

{ "t" : 1000, "i" : 16 }

{ "_id" : 38531058 }

-->>

{ "_id" : 43179696 }

on : brussels

{ "t" : 1000, "i" : 17 }

{ "_id" : 43179696 }

-->>

{ "_id" : 49063836 }

on : brussels

{ "t" : 1000, "i" : 18 }

{ "_id" : 49063836 }

-->>

{ "_id" : 53239611 }

on : brussels

{ "t" : 1000, "i" : 19 }

{ "_id" : 53239611 }

-->>

{ "_id" : 58319948 }

on : brussels

{ "t" : 1000, "i" : 20 }

{ "_id" : 58319948 }

-->>

{ "_id" : 64648683 }

on : brussels

{ "t" : 1000, "i" : 21 }

{ "_id" : 64648683 }

-->>

{ "_id" : 74019617 }

on : brussels

{ "t" : 1000, "i" : 22 }

{ "_id" : 74019617 }

-->> { "_id" :

{ $maxKey : 1 }

} on : brussels

{ "t" : 12000, "i" : 1 }

Full changelog is here: http://pastebin.com/JvyME4dD
Changes about vk_users are in the end.