Publishers of reference material sometimes add false or trap information as a way to catch others who steal their material. Mailing lists often include trap addresses to catch people who rent mailing lists and use them more than once without paying for multiple uses. Maps sometimes add fake streets. Old books of logarithmic tables included intentionally wrong numbers (crazy!). Online trivia contests ask fake questions with searchable fake answers on shill websites to catch cheaters who use the web to look up answers. And dictionaries include made-up words. Here's an example of the latter from Greg Ross' Futility Closet. The 2001 edition of the New Oxford American Dictionary contains a fake word, esquivalience, defined as "the willful avoidance of one's official responsibilities; the shirking of duties."
Sure enough, the word turned up at Dictionary.com (it's since been taken down), which cited Webster's New Millennium Dictionary, and it currently has three definitions on Google Dictionary.
At what point does a fake word become real? NOAD editor Christine Lindberg, who invented this one, told theChicago Tribune that she finds herself using it regularly. "I especially like the critical, judgmental tone I can get out of it: 'Those esquivalient little wretches.' Sounds literate and nasty all in one breath. I like that."