Mischievous is the dictionary-approved spelling of the adjective meaning causing mischief. But the misspelled mischievious, with that extra i toward the end, is so common that some dictionaries have listed it as a variant.
The misspelling is not new. The OED lists instances of mischievious going back to the 17th century, and a Google Books search reveals a few thousand instances from before 1920. So while mischievous is certainly the standard spelling and is safer in formal writing and school papers, using mischievious is not a serious error, especially in informal contexts.
Examples
The misspelled form is rare in 21st-century published writing because spell check catches it, but it does appear from time to time, even in edited publications—for example:
We were good friends, and I will always remember him for the warm, sincere and yet mischievious grin. [Morning Sentinel]
She remembered his mischievious side … [The Star Ledger]
But mischievous is far more common in editorially scrupulous publications—for example:
It is Dionysus, the mischievous god of revelry. [Financial Times]
My guess is Ms. Shemy will find some mischievous ways to make it work. [New York Times]
Tongue-in-bearded-cheek, Ben Hudson has a typically mischievous take on where his retirement will leave the Western Bulldogs. [Sydney Morning Herald]
