Something I wrote to amuse the junior front end developers here:

    document.write('Error: Script not found.');
    var node = document.currentScript;
    if (node.parentNode) { node.parentNode.removeChild(node); }

Pop that in a JS file called something like jQuery.min.js and add it to an HTML page with the usual <script src="/js/jQuery.min.js"></script>. It'll run when the page loads, add the line of text to the page, and then it'll remove it's own <script> tag so there's no reference to it in the DOM (in relatively modern browsers) if you view the source. It's easy to debug by watching the network traffic, but it caused a few scratched heads for a little while.