“undefined” randomly appended in 1% of requested urls on my website since 12 june 2012

Question

Since 12 june 2012 11:20 TU, I see very weirds errors in my varnish/apache logs.

Sometimes, when an user has requested one page, several seconds later I see a simila

Answer 1

I had a similar problem (but with /null 404 errors in the console) that @andrew-martinez's answer helped me to resolve.

Turns out that I was using img tags with an empty src field:

My idea was to prevent browser from loading the image at page load to manually load later by setting the src attribute from the data-src attribute with javascript (lazy loading). But when combined with iDangerous Swiper, that method caused the error.