When a config server is down, new databases (correctly) cannot be created via mongos. However, the error message given is not helpful :

m40002| Thu Nov 3 15:43:24 [conn10] end connection 127.0.0.1:50362 (20 connections now open)
m40000| Thu Nov 3 15:43:24 [conn10] end connection 127.0.0.1:53081 (20 connections now open)
Thu Nov 3 15:43:24 exec error: shell/query.js:118 error: {
"$err" : "SyncClusterConnection::udpate prepare failed: 10276 DBClientBase::findN: transport error: ubuntu:40001 ns: admin.$cmd query:

ubuntu:40001:{}",
"code" : 8005
}
throw "error: " + tojson( ret );
^