Value::compare does not compare heterogenous data types (however the different numeric types can be compared).

In particular this can cause issues if a value may or may not be within an array (even if it's a nested field).

        // CW TODO for now, only compare like values
        uassert(16016, str::stream() <<
                "can't compare values of BSON types " << lType <<
                " and " << rType,
                lType == rType);

This can cause assertions when trying to $sort different data types for example:

c = db.c;
c.drop();

c.save( { a:1 } );
c.save( { a:true } );

printjson( c.aggregate( { $sort:{ a:1 } } ) );

And in particular it can also cause problems when some fields are within arrays (even nested):

c = db.c;
c.drop();

c.save( { a:{ b:1 } } );
c.save( { a:[ { b:2 } ] } );

printjson( c.aggregate( { $sort:{ 'a.b':1 } } ) );

Observed behavior: An assertion is raised when an attempt is made to sort a field containing different data types in different documents.
Expected behavior: The results are sorted, without an assertion, in spite of the different data types.