Ask HN: Why do common JavaScript errors still happen in major production sites?

I read HN every day and love the discourse on the advanced, mature, wide-ranging and ever-evolving toolsets for modern front-end development. All the techniques that exist now to guard against the simple JS errors of yesteryears. Yet I wonder if I am in the minority and even the largest companies in the world are not paying attention to any of these techniques. Because I often see things like this, today (and every day recently) when I log into my cell phone provider's web site (Vodafone), and am greeted with this large, prominent widget first thing at the top of the page:

http://imgur.com/a/ZbvKv

How is stuff like this still so common in modern web development?