As far as anybody can tell, if a split happens while mongodump is dumping a config server, something bad is likely to happen:

• if mongodump uses the _id index, then there will be multiple chunks with overlapping shard key value ranges,
• if mongodump happens to traverse the collection in natural order, the dump output might have overlapping or missing ranges of shard key values

So we ought to do something to make this work. Offhand, either these might do

• tell people to run config servers with --master and to dump them with --oplog
• add a way to prevent splitting analogous to the way we can prevent balancing

Maybe there are other ways to do it, too.