This will be an error you would see when trying to generate entities or documents using:
app/console doctrine:mongodb:generate:documents YourPathToBundle
Actually the error would also suggest if you have mapping annotation error.
Mostly this could arise from mapping error as suggested.. check if you have @MongoDbDocument in the case of document or the respective @ORMEntity that would tell what that Plain Old Php Object is representing..
btw, the MongoDb and ORM are the aliases, so it could different on your setting..