I was taking a look to the JUG section of java.sun.com and went to the search form . I just clicked the Search button without entering form data and voilˆ! I got an exception, not only telling the system was not througly tested but also that the underlying database is Oracle (see the “ORA” code)
java.sql.SQLException: ORA-00936: missing expression
This made me recall a conference about search engines where I was told that the most searched word was… the empty string! So don’t underestimate user dumbness 😉
Carlos Sanchez's Weblog 
It used to be that if you denied cookies from java.sun.com, the home page would display a nice NPE stack trace. It was then changed to show just “0” (their quotes).
A certain company’s product had a tendency to display links to .bash_history files if the search string was empty. I prefer the exception.