Although incidentally has a primary meaning of happening in connection with something else, it is more often used to mean "apart from the main point" and "by the way": "Incidentally, I forgot this minor matter." "Whether you come or stay is incidentally unimportant." Although incidently was once in good use, it no longer appears in the language and is considered an illiteracy. Incidentally should be spelled and pronounced with five syllables.
Dictionary of problem words and expressions. Harry Shaw. 1975.