Although mischievious isn't listed as a variant spelling in Merriam-Webster's Eleventh Collegiate Dictionary, you could make the case that it is one and that people who say "mis-CHEEV-ee-us" are pronouncing that variation of the word correctly, rather than pronouncing mischievous incorrectly. Here is an Ngram chart tracking occurrences of mischievious in the Google Books database across the years 1600–2000:
The two earliest instances of the spelling mischievious that a Google Books search finds are from the 1660s. From Thomas Wilson & John Bagwell, A Complete Christian Dictionary (1661):
Malicious ] 2 Joh. 10. Naughty, mischievious, evill, wicked.
Nevertheless, that same dictionary also has this entry:
Mischievous ] One devising evill, Prov. 24. 8.
From Richard Head, The English Rogue: Described, in the Life of Meriton Latroon (1665):
The next day I went into Lincolns-Inn-fields, where I saw a company of Rogues, cheats, Pick-pockets, &c, playing at Pidgeon holes (a game much practised there, and in More-fields, by such mischievious and lazie Rascals) ...
For the period 1702-1720, Google Books finds twelve unique matches for mischievious—enough to suggest that the spelling may have reflected a then-current variant pronunciation, and not merely a series of typographical errors. On the other hand, it is undeniable that a person who fully intends to type mischievous may type mischievious by mistake.