Some mistake of signup:

Having a sign-in in the first place
Requiring sign-in too soon
Not stating the benefits to registering
Hiding the sign-in button
Not making "Create a New Account" or "Forgot Your Password" a button or link
Not providing sign-in opportunities at key locations
Asking for too much information when registering
Not telling users how you'll use their information
Not telling the users the requirements for username and password up front
Requiring stricter password requirements than the NSA
Using challenge questions they won't remember in a year
Not returning users to their desired objective
Not explaining if it's the username or password they got wrong
Not putting a register link when the sign-in is in error
Not giving the user a non-email solution to recover their password
Requiring more than one element when recovering password

http://jchyip.blogspot.com/2008/04/16-design-mistakes-for-account-sign-in.html