.... Say what?
There is a little easter egg, though some may not think it's so innocent, in the ever wonderful web browser Internet Explorer. That is, if you are a web dev and run a domain name, you'd best not put a underscore "_" in it! No kidding Jack! IE will ignore cookies for the domain! Apparently it is not RFC-Compliant. And now you know ;-)
To promote interoperability and consistent with long-standing
guidance about conservative use of the DNS in naming and applications
(e.g., see section 2.3.1 of the base DNS document, RFC1035 ),
characters outside the set of alphas, digits, and hyphen MUST NOT
appear in domain name labels for SMTP clients or servers. In
particular, the underscore character is not permitted. SMTP servers
that receive a command in which invalid character codes have been
employed, and for which there are no other reasons for rejection,
MUST reject that command with a 501 response.