Who you gonna call? And how you gonna look up their phone number?
Chris writes about yuureimoji:
[T]here are apparently certain characters called 幽霊文字 ("ghost characters") that have no readings, meanings, or examples of use. Even if you look them up in a dictionary you get definitions like 意義未詳 (reading and meaning unknown). Examples of these ghost characters are 暃 and 碵.
They all come from the JIS set, which is a set of characters that are standard for computer terminals to display. Apparently during the compliation of the JIS set, some characters that weren't actually characters got onto the list accidentally -- either because they were miswritten versions of actual characters or the compilers misread certain kanji.
So why do they appear in dictionaries? ...
This seems like an eminently sensible development to me. People are always complaining about kanji having multiple readings, saying that one reading per kanji would be preferable. What, then, could be better than kanji that have no readings? (Borges would loved it, too.)
But are they all really mistakes and nothing more? Japan has a long history of literacy, and only a very small portion of that had anything to do with computers, and snap-together writing systems are wide open to abuse innovation. It doesn't matter whether a new character is created intentionally or not -- if enough people use it, the Blue Fairy of descriptivism makes it a Real Character.
Here's Chris's list, for reference:
粫 挧 橸 膤 袮 閠 妛 暃 椦 軅 鵈 恷 碵 駲 墸 壥 彁 蟐
A couple of these are about as real as you can get: 粫 was apparently in use as part of a place name; 挧 is a Chinese surname (pronunced /yŭ/, it seems); 膤 is, according to my dictionary, part of a placename in Kumamoto prefecture: 膤割 (Yukiwari).
袮 is allegedly a simplified version of 禰, meaning "one's father's mausoleum", and this is quite believable because after all 尓 is a simplified version of 爾. And I don't see any real reason to doubt 軅's validity as a Japan-made variant on 軈 -- all it takes is one influential writer to omit that 心 at the bottom and the dot on top, and a new character is born.
On the other hand, even the JIS bigwigs admit that 妛 and 椦 are indeed just mistakes.
This is the kind of topic that Google can't really do justice to, sadly. Which is why I propose we launch an all-Wikipedia investigation! What could possibly go wrong?
Update! A helpful follow-up from Chris.
amida:
Matt: In Chinese, 袮 is sometimes used by Christians as a second-person ("second-deity"?) pronoun to refer to God. Standard 你 has a person-radical and that just wouldn't do, would it? Better replace it with that old religion-friendly 示. Both are read "ni3." There's also 祂 for the um, third-entity pronoun, taking the place of 他, 她, 牠, and 它.