Form Validation: using set_rules as conditional?

[eluser]bpdp[/eluser]
Thanks for all the replies. That kinda works but won't work for me. depending on how I do it I either get a "Unable to access an error message corresponding to your field name." error. or the email field doesn't repopulate when the user is returned to the form. I can't believe that this is not a common problem, seems like a very basic thing.