If you're as old as me, you'll possibly have childhood memories of The Mr Men voiced by Arthur Lowe. (Absolutely the best thing about being off school sick and being able to watch lunchtime TV). Mischievous was always pronounced as mischiev-ee-us.
I remember my P3 teacher correcting me and I was convinced she was wrong because, well, it was the telly, and surely everything on the telly was correct? 