Instapaper requires an account to save content to “Read Later”. On the account creation page, leaving the password field blank results in an error alert message.
So, a password is required. Later, when you use the Instapaper bookmarklet, if you are logged out, then you will be requested to login. The login form states, explicitly, that you probably don’t have a password.
I wonder if a lot of people end up making duplicate accounts based on this simple form field help text? I think this is a usability oversight and would remove the help text since it misinforms users.