I regularly come across sites that only work with "www.". Common with university sites that use the subdomain hierarchy a lot. For extra fun, make the behavior reversed depending on if you are inside or outside their network.
Same with my uni. And then you have sites that only work with, and others that only work without the www.
But it makes sense. the first subdomain before the uni domain specifies the faculty, many of which have their own datacenters. Then many of those have yet their own servers in their network, and often www. is one added later on.
Oh yeah. And then someone enables HSTS and a subdomain doesn't support HTTPS, so now you have to keep a browser around that never ever is allowed to contact the parent site...
