Hey all,
I sent in my first 5 applications (Columbia, Northwestern, Chicago, Purdue, Penn State) with a huge mistake (a half completed sentence that ended a paragraph) in my writing sample. I only discovered the issue a few days ago, and thought all hope was lost with those programs.
But all hope isn't lost. If you've made a mistake like I did, some programs are willing to let you fix the issue. Just contact the graduate admissions officer via email and explain the mistake.
I hope this helps someone.