Yes, the default Recent Posts widget.
The error is persisting for me (on two different installations) with TwentyTwelve, and still exists after deactivating all plugins, including deleting all mu-plugins.
After some testing, I believe that there are a few more conditions that need to be met in order to generate this behavior, so if you just made a blank new site, you wouldn't experience it.
If the recent posts widget would contain a published post without a valid "_ppp" query var, then the widget is rendering the "The link has expired!" error and wp_die()ing. (line 340)
If I trash those 'offending' posts, then I don't experience the error, because the widget is just not rendered as there are no Recent Posts to display.
If I just publish a new post, then the widget breaks again because this post SHOULD show up in recent posts, but it doesn't have the valid ppp nonce.
Anything else you can suggest I do to try to help troubleshoot what's going on? Thanks again