In one of my web application projects I wanted to create some modal popups that would collect user input and validate. I ran into an error using required field validators and regular expression validators.
The modal popup would show as normal and if the validators would trigger on bad input. Once a validator was triggered, if the user cancelled the modal popup they would no longer be able to activate any controls on the page.
Always put your validation controls in a validation group, this will prevent a few headaches you may run into. If you have multiple modal popup forms, put each forms validation controls in their own validation groups.