This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
In Struts version 1.2.9 and above, you will get this error unless you specify cancellable="true" in the action path definition in your struts-config.xml file. Since the cancel action bypasses validation, this was a security hole. Now it only works if you specifically configur it to work.