db.drop();
db.foo.insert({a:{$ref: "bar", $id: 1, $db:"other"}});

// any of the below fail
db.foo.distinct("a.$db");
db.foo.distinct("a.$ref");
db.foo.distinct("a.$id");

Thu Feb 06 09:23:44.489 distinct failed: {
        "errmsg" : "exception: Can't get runner for query {}",
        "code" : 17216,
        "ok" : 0
} at src/mongo/shell/collection.js:879