For the last few days there has been a problem with one of the Add servers and its choice of advert source, have not been able to tie down which or what, resultant page freezes, page not found pages presented and various browser notifications appear to depend upon which browser(version) and operating system you are using and how it deals with an erroneous url.
You may see the page freeze, you may see a Page not found window or a popup message that an Add called by the page can't be loaded.
An annoying niggle courtesy of Google.
You could try cleaning unwanted cookies off the system to see if that stops the occurrence.