Observed behavior: It is possible to create a $group aggregate field with a field name containing a dot.
Expected behavior: No dots allowed, uassert instead.

I think the dots might cause a problem when a group merger on mongos attempts to access fields set in the mongod results, but I haven't tested that specifically.

Test:

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

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

printjson( c.aggregate( { $group:{ _id:0, 'z.x':{ $push:'$a' } } } ) );

Result:

{
	"result" : [
		{
			"_id" : 0,
			"z.x" : [
				2,
				1
			]
		}
	],
	"ok" : 1
}