> db.foo.drop()
true
> for (var i = 0; i < 20001; i++) { db.foo.insert({a: i}); }
> db.runCommand({group: {ns: "foo", key: {a: 1}, $reduce: function(a, b) {}, initial: {}}})
{ "ok" : 0, "errmsg" : "EOO", "code" : 2 }