The Cupertino effect is the tendency of a spellchecker to suggest inappropriate words to replace misspelled words and words not in its dictionary.
The origin of the term is that the spelling "cooperation" was often changed to "Cupertino" by older spellcheckers with dictionaries containing only the hyphenated form "co-operation". (Cupertino is the home of Apple Inc., and thus would be in most computer spelling dictionaries.) Users sometimes clicked "Change All" without checking whether the spellchecker's first suggestion was correct to begin with, resulting in even official documents with phrases like "as well as valuable experience in international Cupertino".
More generally, "Cupertino Effect" means failing to check that a suggested word is appropriate. A common case is "definately" being changed to "defiantly" instead of "definitely". Benjamin Zimmer of Thinkmap, Inc. and the University of Pennsylvania has collected many examples of similar errors, including DeMeco Ryans as "Demerol" (in the New York Times), Voldemort as "Voltmeter" (in the Denver Post) and the Muttahida Qaumi Movement as "Muttonhead Quail".
The user need not always select an incorrect word for it to appear in the document; in WordPerfect 9 with factory default settings, any unrecognized word that was close enough to exactly one known word was automatically replaced with that word. Smartphones with dictionary supported virtual keyboards automatically replace possible mistakes with dictionary words.
Zimmer suggests that a possible misspelling that produced the Cupertino effect is "cooperatino". A member of the European Union translation service reports that the Cupertino change can happen to the word "cooperation" if the word processor's custom dictionary only has the hyphenated form "co-operation", and this was verified to have occurred using the spellchecker on an older version of Outlook Express.Cupertino has been present in Microsoft's custom dictionaries since at least 1989 (when Word 4 for Mac was released).
More sophisticated techniques for spell-checking in modern programs mean that the effect is much less of a problem than it used to be.