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It's a DNS resolution issue. Underscores in domain names are invalid, but some DNS host software still resolves them, while others stick to the spec.

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I'd call this a fruit of this kind of DNS abuse. Yeah, OK, it looks nice, x.usesthis.com or whatever it is, but it forces you to make a new DNS lookup for each page, instead of using cached www.usesthis.com/something. And, as we can see, you can even make the address invalid. It's quite sad. :-/

Web content that is invalid according to one or more standards? Surely you must be joking.

Underscores have been allowed in domain names for quite some time. DNS based Service Discovery even requires their use.

EDIT: If I'm interpreting the specs correctly, underscores are fine in a domain name but not in a hostname.

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