gistfile1.js

// one-liner
javascript:(function(src, count) { var img = new Image || document.createElement('img'); img.addEventListener('error', function() { if (!count) img.src = src; else alert('PASS'); count = 1; }, false); img.src = src; })('data:text/plain,fail')

// prettified
javascript:(function(src, count) {
  var img = new Image || document.createElement('img');
  img.addEventListener('error'
    ,function() {
      if (!count)
        img.src = src;
      else
        alert('PASS');
      count = 1;
     }
     ,false
  );
  img.src = src;
})('data:text/plain,fail')