I was trying to buy tickets online from a website where I was not sure if I had purchased before. So I acted like I was a new account. Upon checkout, I got this message:
Well, which is it? Your system MUST know. Your system has to know if there is an account with this email address. Your system KNOWS if the problem is that I entered an invalid username/password combination. You know what the problem is. I don’t. Now I am not sure how to fix it. If I try to fix my username/password combination, but the real problem is that I have an account (and you want me to log in), then I will waste my time trying to fix my form.
FAIL. Error messages need to tell the user in plain, obvious language what is wrong AND how to fix it. It would have been a better experience if the message had come back that there was already an account with my email address. It would be even better if it then showed a login form AND a forgot password interface. Boom. Without extra clicks, I have whatever I need right there.