Saves your bacon when you're using jsbin.com with OS X and try to Cmd+Left to go to the beginning of the line.

permission_to_leave.js

// ==UserScript==
// @name        Permission to Leave
// @namespace   permission.to.leave
// @description Prompts before leaving the page. Good for OS X users who press Cmd+Left while editing text to go to the start of the line, only to find the browser goes back a page instead. 
// @version     1.0
// @grant       none
// @include     http://edit.script.and
// @include     https://add.target.sites.here
// @include     https://twitter.com
// ==/UserScript==
window.addEventListener('beforeunload', function(e) {
  var confirmationMessage = "Brought to you by the Permission to Leave user script.";
  (e || window.event).returnValue = confirmationMessage;
  return confirmationMessage;
});