When performing your reconnaissance, you may come across parts of web applications that will allow you to determine whether or not certain usernames are valid. A prime example of this will be a page that allows you to request a password reset when you have forgotten your password. For instance, if the page asks that you enter your username in order to have a password reset, it may give different responses depending on whether or not a user with that username exists. So, if a username doesn't exist, the page may respond with Username not found, or something similar. However, if the username does exist, it may redirect you to the login page and inform you that Password reset instructions have been sent to your registered email address.