From Candybar 3 FAQ
http://www.panic.com/candybar/support.html
Why can I no longer change "Application Extras"?
CandyBar 2 let you modify icons inside of an application, including document icons. But things are much different in Leopard: a new security feature in 10.5 adds the ability to validate the integrity of applications by scanning their contents, and could potentially cause system problems if internal icons are modified. We try to play it safe — as a result, this feature has been removed in CandyBar 3. We're very sorry for the inconvenience.
Just a reminder if you use Leopard firewall.