renaming a->b if a DNE is not an error i guess. but the following is as it isn't allowed, yet no error set.
> t.find()
{ "_id" : ObjectId("4df4da0dd5184551b584cf37"), "a" :

}
> t.update({},{$rename:{"a.b":"top"}}); db.getLastErrorObj()
{
"updatedExisting" : true,
"n" : 1,
"connectionId" : 2,
"err" : null,
"ok" : 1
}
> t.find()
{ "_id" : ObjectId("4df4da0dd5184551b584cf37"), "a" :

}