Mischievous

In formal British English, the word mischievous is pronounced MIS-chi-vus, IPA: /ˈmɪs tʃə (or ɪ) vəs/. It is sometimes pronounced as if it were written mischievious (mis-CHEE-vee-us, IPA: /mɪs ˈtʃiː vɪ əs/) - this is the majority pronunciation in American English - and this is seen in writing too. It's wrong in both spoken and written formal English in the UK. (OED lists as many as 82 spellings of the word, so it must be recognized as hard to spell. It isn't necessarily so - as long as you say it correctly, you'll probably get the '-i-' in the right place.) Some RP speakers make the second vowel almost the same as the first: 'Mis-chif', IPA: /ˈmɪs tʃɪf/, in the noun and IPA: /ˈmɪs tʃɪv əs/ in the adjective.

Mischievous is formed from the noun mischief, which has two syllables, and the suffix -ous, which has one. So the British (RP) pronunciation has some logic on its side.

Compare Grievous, and, for the non-standard pronunciation, devious and previous, which are both pronounced with the ending '-y-ous' (/-ɪ əs/).