In MongoDB 2.0.7 it is possible to update() a document and set an invalid field name starting with $:

> db.test.insert({_id: "foo"})
> db.test.update({_id: "foo"}, {$set: {$set: "bar"}})
> db.test.find({_id: "foo"})

If you mongodump and then mongorestore this database, documents with invalid field names are silently ignored (even with `mongorestore -vvvvv`).

In MongoDB 2.2.0rc2 the attempt to update() does throw an exception:

Modified field name may not start with $

.. but mongorestore still does not report any errors for skipped documents.