Following the pageflow tutorial, everything is fine until the validation application which has a minor bug (just looks bad if letting a user play with it in a demo).
Get to this stage:
Deploy the validation app to Tomcat.
Go through all the pages one time with valid data until you end up on this page:
Now, go back to page2.do by clicking browser back button.
Now enter INVALID data (age = 200 for example).
Instead of staying on page2 with the invalid data message you end up being sent through to processData with empty data.
If you go through the flow without ever using the back button, you don't get this type of problem...