There are some accounts which you can't really use because of the way things are set up.
Clicking on the userhead here --> user will not take you to "user"'s journal but to "profile"'s journal - because user.livejournal.com/profile redirects to users.livejournal.com/profile, which redirects to profile.livejournal.com. Compare also http://community.livejournal.com/ to http://community.livejournal.com/profile. Poor thecommunity. (Edit: "community" was apparently renamed to "thecommunity". The "lj user" tag knows that, but the links don't redirect. No big loss, though: oy, what a cesspool of spam that place seems to be.)
www is also in limbo. I guess m was registered defensively before them mobile version of the site was announced; at least, it appears not to be a regular user's account but instead a placeholder along the lines of exampleusername. (m's profile works; their journal doesn't since that URL is the mobile site itself.)
And because of the @livejournal.com email addresses, abuse and support were also in a weird position of not being able to use their @lj address since those email addresses were handled specially. (I'm not sure whether they still are, or whether those usernames have been retired somehow.) lj_feedback too, presumably. (Edit: Guess "feedback" redirects to "lj_feedback" now. And "support"'s profile doesn't work but instead redirects to the support area. "abuse" seems to be a normal account, though.)
Also, there's a user status, and you can read their entries on your friends page but can't click through to them, because status.livejournal.com is handled specially.
The newbie lounge (anyone remember that?!) was also weird in that respect: there were journal entries but you couldn't see them because the journal style always showed a static page. (I think that users to be newmembers, but that account seems to be something different these days; simply a normal community.)
And I think 0 (that's <lj user="0">) was (is?) broken because of checks in some places considering "0" to be "false" in the Perl sense, so some code thinks the journal name is essentially blank or something.
Also, anyone remember test? Used to be a test account with a well-known password, intended for client authors to test their programs, back when there were still invite codes. I think it got suspended after a while, once codes had been removed for several years. There seems to be an account there again, but presumably it's a different one (someone who had renamed to test).
Dreamwidth seems to have a large-ish "reserved names" list preventing you from registering "www" or "user" or similar things.
Does anyone know any other similarly broken accounts?