from Garner's Usage Tip of the Day
"Mischievous" /MiS-chuh-vuhs/ is so spelled.
"Mischievious" is a common misspelling and mispronunciation /mis-CHEE-vee-uhs/ -- e.g.:
o "I could not imagine them driving, getting mouthy, moody or mischievious [read 'mischievous'], let alone going to drinking parties at the homes of friends whose parents were out of town." Eleanor Mallet, "The Tranquility of School Age," Plain Dealer (Cleveland), 25 Feb. 1995.
o "Mayan Indians considered this place hell's fun house, inhabited by mischievious [read 'mischievous'] gods who had to be soothed with heaping food bowls and the occasional human sacrifice." Judith Wynn, "Lodge Guests Settle In Among Tropical Wildlife," Boston Herald, 26 Dec. 2002.