Right now if someone attempts to use an aggregation expression inside a query or inside a $match, they will see results different from what they expect (and no error):

t = db.t;
t.drop();

t.save( { a:1, b:5 } );
printjson( t.find( { a:1, b:{ $gt:{ $add:[ 1, 1 ] } } } ).toArray() );
printjson( t.aggregate( { $match:{ a:1, b:{ $gt:{ $add:[ 1, 1 ] } } } } ) );

In both cases

is treated as a document to compare against, not an expression to be evaluated.