When you've made changes to the buffer, vi won't let you quit normally, such as with :q. One of the toughest situations every vi user faces is the dreaded E37: no write since last change (add ! to override) message. This error can be fixed only by using the correct additional ! character. To exit a file that is read-only or that you don't want to save the changes to, you must use the keystrokes :q!. This is known as qbang or quit dammit.