A recent ticket (00664772) with the mongodb support team confirm that the following case is not handled by the Java driver:

class Foo {
int x;
int y;
}

I can successfully write and read it. However, if the Document has extra fields, it cannot be deserialized and no option exists to allow this behavior:

{ x:123, y:456, z:789 } // why can't I ignore 'z'?

My suggestion is, provide an annotation to support this behavior:
@BsonIgnoreUnmappedFields
class Foo {
int x;
int y;
}