Adding error-handler to ajax produces additional request

I see something strange in console (http://localhost:3001/errajax) I just added a stub to ajax object and datatables began to produce an additional json-request like: GET https://datatbl.herokuapp.com/errajax?_=1448540064717

var table = $('#users').DataTable( {

    ajax: { error:  function( e, settings, techNote, message ) {} } 
});

Is it a bug or one more time my mistake?