JavaScript performance is always on the mind of engineers as they chip away at average page load times.  But occasionally a JavaScript failure is so bad that it bubbles up to the attention of the average web user. That’s exactly what happened last Friday, when the “”Login with Facebook”” script took down a number of sites completely. Any users who happened to be logged into Facebook (so: many, many users) were redirected away from sites like CNN, Gawker, MSNBC and others, to be shown a Facebook error page. (See video below courtesy of AllThingsD)