A common and disgusting mispronunciacion and/or spelling of the word mischievous. Mischievous is pronounced "miss-CHI-vus" and there is no "ious" about it. Some people insist that mischievious is an "alternate pronunciation," but only in the way that irregardless is a synonym for regardless--that is to say, only for fucking retards. Some argue that it is the evolution of language, but changes like this are driven by the illiterate and the ignorant, and it is careless to embrace them. And embarrassing.
"I'm intelligent and mischievious!" (oxymoron)
"Mischievous is not like devious, it's a completely different word you dumb shit." --Me
"Since everyone says mischievious it's not wrong." --what my dumbass friends insist if the subject comes up over strong drink.