The worst thing you can do is to confuse your users.
I just stopped by the Pidgin Trac site to enter a feature request. Not remembering my password, I used the "reset my password" feature, which sets a new random password and emails it to me. Excellent.
Now, Trac is fairly clever about logins. (I say fairly, because while this may seem simple enough, the devil is truly in the details.) When you click the login link, it remembers where you were, and redirects you to that page upon successful login. That's very good. Not so good, however, if the last thing you did was to reset your password.
You see, when I had received my new password and used it to log in, Trac redirected me back to the "reset your password" page. That page, being somewhat clever on its own, considers it an error when you try to reset your password while already logged in, and tries to be helpful by recommending that you visit the settings page if you want to change your password.
In the end, the first thing I saw after a successful login was an error message.
Good intentions gone wrong. Close, but no cigar.