I've found a bug I can reproduce reliably. If a newsletter guest signs up, an email is sent for confirmation. Sometimes that email may get caught in the spam folder, and the user doesn't click on it. User then re-requests a guest membership with same name and email. Another email is sent requesting confirmation, with a different confirmation link. If the user clicks on the second email they received, they see the following error. WARNING: constant() [function.constant]: Couldn't find constant _TPL_SENDPWD_NEWGUEST_INCINFO in line 117 of file common.inc.php And the user is not added. If they click on the first email, everything works fine and the guest is added. I suppose the workaround is don't verify emails.
I have also noticed that guests can subscribe the same email address twice. It shows up in the newsletter count on the Newsletter Thread management page. They only show up once on the Newsletter Guest management page. I didn't test whether they receive duplicate emails.
Just throwing the thought out there ..... Maybe setting the index in the table to unique ? Although code to check for duplicates at time of subscription would be more useful and elegant