It is possible to upsert a document with differing _ids on different config servers if the upsert is of the form:

db.coll.update({ _id : { $gt : 1 }, user : 1 }, { $set : { user : 1 } }, { upsert : 1 })

The _id field is present as an expression in the query, so _ids are still autogenerated on upsert. This form should probably just be disallowed to configs.