> db.foo.find()
{ "_id" : ObjectId("5010009b54f3e488821117fc"), "j" : 0, "s" : "abc", "e" : [ { "t" : "a", "c" : 100 } ] }
> db.runCommand( { findAndModify: "foo", query : { s : "abc", j : 0 , 'e.t' : "a" }, update : { $set : { 'e.$.c' : 3 } }  });
{
        "value" : {
                "_id" : ObjectId("5010009b54f3e488821117fc"),
                "j" : 0,
                "s" : "abc",
                "e" : [
                        {
                                "t" : "a",
                                "c" : 100
                        }
                ]
        },
        "errmsg" : "exception: can't append to array using string field name [$]",
        "code" : 13048,
        "ok" : 0
}