I've got a Page showing up in my Assignment Index:
Homework 12, part 1 is an assignment, Homework 12, part 2 is a quiz, and Discussion Schedule is a page.
I don't know how Discussion Schedule got there, or exactly how long ago but I don't know how to get it off my Assignment Index without deleting it: I made a duplicate of the "Discussion Schedule" page within the Pages Index and the duplicate also showed up on my Assignment Index. I then deleted the duplicate from the Assignment Index and it also deleted it from the Pages Index.
Fortunately, this doesn't show up for the students, but it's annoying to have this extraneous page link in the middle of my Assignment Index.