If you try to access a Lotus Sametime server with Internet Explorer 5.5 or higher you get the error message "Cookies not enabled" although you are sure that cookies ARE enabled for this host.
The reason for that is an underscore ("_") in the host name of the Sametime server. The problem is already documented some time ago but I was not aware of this and had a hard time to find this out ....
I am quite sure that somewhere you also get a warning when installing/configuring a server with an underscore in the host name but I did ignore it 🙁 So although there seems no obvious problem with such a name you better should take care of it because of unwanted side effects later.
This btw. is no problem of IBM or Microsoft but it is not supported according to RfC 952.