The breakageThe following webapps are susceptible to “__proto__”-induced breakage:
- The classic – Google Docs: For a while, if you typed in “__proto__” at the beginning of a document in Google Docs then it would hang.
- Current – Twitter: If you click on @__proto__ in a tweet then the profile summary that comes up only has a title bar, but no content. You also get “slow script” dialogs in Firefox. You can try it out in this tweet.
__proto__ is only supported in some browsers, things only break if it is supported. Thus, Firefox exhibits these problems, but Internet Explorer 9 does not. However, __proto__ will become part of ECMAScript 6  which means that all browsers will eventually support it.