Infamous common misspelling of the word liaise.
"Liase" is now actually supplanting the proper spelling, purely because this is the way "liaise" is spelled in Microsoft Word's default spellchecker dictionary. This isn't even a US English vs. British English kind of thing, either, because all US dictionaries I've checked list the word as "liaise" as well.
Most people trust their spellchecker to be right. In this case, it isn't. Because of the ubiquity of Word, however, this misspelling is (regrettably) becoming acceptable--even the norm.
We all know that Microsoft occupies a monopolistic position in the operating system market, and as such bends the computer marketplace to its will. Isn't it interesting how this monopoly can have a distorting effect on the English language as well?