SERVER-106590 relaxed the database lock from MODE_X to MODE_IX in order to allow concurrent migrations on the same DB to be executed. In the cloner code, this means that now a concurrent migration can sneak in and make a WriteConflict exception when it's trying to create the collection. This is an expected behavior, however, during the first execution of the collection creation obj() could've been called on a BSONObjBuilder that is currently being created in another scope. This is a problem for a second execution of the retry loop, because it was previously invalidated.