Yeah, that stuff gets slipped up all the time. It's usually not the writer's fault.

For example (with MLA at least), if I say that "You'd be surprised." (Marvo 9), and I didn't put an entry for Marvo in my works cited page, I'd be plagiarizing. Or, if I just said "You'd be surprised", that's also plagiarizing. It gets really specific. For my last essay I got it handed back because of errors on the works cited page where I would have lost so many points I would have been better off just not handing it in.

Thankfully, she gave me a chance to fix it because I handed it in early.