Hi Melody,
It sounds like you have set the "Signup Success" custom page to forward to a URL on your site (instead of using the HTML entered in LMP and adding dynamic error messages). Is that right? Can you tell me exactly what you have set up and where?
Each "set" of Custom HTML pages can be applied to one or more lists. You add and modify sets from the Custom Page, then the set each list uses is chosen on the List Settings page. Could this be causing some confusion?
Regards