This is a head's up for Simon or Hans.
There's some weirdness happening with some Chrome browsers. It's not all Chrome; and the error isn't confined to Wizzley. There's more information here.
However, I have two friends for which this is happening on Wizzley. They can see the profile pages just fine, but as soon as they click on an article, the URL gets a random /undefined added to the URL. This naturally sends them straight to a 404 page.
It seems that the issue may be a third party plug-in in Chrome, thus nothing to do with Wizzley. But both of my friends say they haven't got that plug-in. It's happening to both of them on a number of websites, not just ours.