Default Mongo exception.
This covers a bunch of different error conditions that may eventually be moved to more specific exceptions, but will always extend MongoException.
non-utf8 string
This error occurs if you attempt to send a non-utf8 string to the database. All strings going into the database should be UTF8. See php.ini options for the transition option of quieting this exception.
invalid regex
The regex you passed in does not match the form /regex/flags.
The MongoSomething object has not been correctly initialized by its constructor
Probably your Mongo object is not connected to a database server.