The $ operator cannot be used in projections if there is more than one array field in the corresponding query. (http://docs.mongodb.org/manual/reference/operator/projection/positional/#array-field-limitations)

This seems counter-intuitive since in the projection, the $ operator occurs in an array field name and so it should be unambiguous to which array it refers.

I'd suggest therefore to remove this limitation and allow the $ operator even if there is more than one array field in the corresponding query.