Chris Kern has an entry on “ghost characters”:
Proving once again that the Japanese writing system is supremely screwed up, there 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.
Matt of No-sword, in his post on the subject, shows that some of the characters are real, if obscure, but adds “even the JIS bigwigs admit that 妛 and 椦 are indeed just mistakes.” Something of a parallel to ghost words in English.