The second way that a non-word can show up in a dictionary is through someone copying erroneous words from another source. The history of dictionaries contains many incidents of unscrupulous dictionary-compilers stealing entire word entries from other dictionaries.
And this brings us to the third way that fictitious words can show up in a dictionary: They can be deliberately put there. It is common practice among dictionary publishers to insert completely made-up words so that when someone plagiarizes their work, they can catch them red-handed.
In 2005, one of the fake words in the New Oxford American Dictionary was revealed to be "esquivalience" which supposedly meant "the willful avoidance of one's official responsibilities".
In 1903, a dictionary of musical terms was published, and at the very end was this definition:
This definition was reused in Mrs. Byrne's Dictionary of Unusual, Obscure, and Preposterous Words: Gathered from Numerous and Diverse Authoritative Sources which was published in 1974, but with an alternative (and seemingly, completely fabricated) pronunciation.
zzxjoanw (shaw). Maori. 1. Drum. 2. Fife. 3. Conclusion.
If the definition "conclusion" and the incongruous pronunciation weren't evidence enough, a few other facts should have tipped people off that this word is a hoax: a) The Maori language does not contain the letters "j", "x", and "z". b) All Maori words end in a vowel. c) In traditional Maori culture, there are no drums!
Other compilers of information also introduce false information for the same reason. Phone book publishers have included fake names and phone numbers. Encyclopedias are published with false entries (like the New Columbia Encyclopedia's entry on Lillian Virginia Mountweazel, a fountain designer and photographer (famous for photographing mailboxes) who died "in an explosion while on assignment for Combustibles magazine". And map-makers insert fake streets (called "trap streets") or draw their streams as if they were slightly squigglier than they actually are.
What lesson can we draw from all this? Be skeptical. Don't just accept everything you read. Not even in dictionaries. Not even on this blog! And certainly don't believe words when they start with "zzx". That would be dordish.