This only works on the web.
If you put something like this in Input Validation on a field:
@If( @ThisValue = "dontpanic" ;
@Failure( "[http://dontpanic82.blogspot.com/]" ) ;
When the user writes "dontpanic" in a field with this input validation, and try to save the document, they will be redirected to my blog.
The only use-case I can think of at the top of my head is lookup-based login-forms.
..code to check username/password..
@If( authenticated ;
@Failure( "[/" + @WebDbName + "/login?OpenForm&error=wrong_username]" )
Another use case is maybe to limit too wide searches. A customer recently searched for *ia*, in a several CMS' and thought that the search didn't work (when I tested, the search took 3-4 minutes to complete).
If you can think of/have other ways to make use of this, please post it in the comments..