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.

So why do they appear in dictionaries? It appears to be solely as a marketing tactic; they want to be able to put "contains all JIS kanji" in their advertisements to be competitive, but that means they even have to include these fake characters.

EDIT:
This appears to be the full set of ghost characters:
穃粫挧橸膤袮閠妛暃椦軅鵈恷碵駲墸壥彁蟐
Some dictionaries give readings for them, or meanings, but they are just guessing (or using readings/meanings of characters that look a lot like them -- for instance, KANJIDIC gives 軅 the reading of "yagate" but that's just taken from 軈.)