Just happened a couple of times in a row, both screens had the EE Samsung S7 advert. The culprit perhaps? I also noticed that there's a 'don't ask me again' option on the pop up. Must remember to select that next time it pops up.
EDIT: Nope, just happened on a page without that ad, too.
Every so often I keep getting an unresponsive script alert:
A script on this page may be busy, or it may have stopped responding. You can stop the script now, open the script in the debugger, or let the script continue.
Script: https://code.jquery.com/jquery-2.1.4.min.js:2
(windows 10...