“Invalid Username or Password”: a useless security measure (2014)
> 99.9% of websites on the Internet will only let you create one account for each email address. So if you want to see if an email address has an account, try signing up for a new account with the same email address.This is not true if the signup flow is implemented correctly. Signing up for an account should always respond with the same message "we sent an email for you to confirm your account signup". The owner of that email address then receives an email - either 1) normal signup process, or 2) "did you just try to sign up? You already have a valid account for this email address."This way you cannot tell via the signup web form alone whether an account exists or not. You need to have access to the email address.