Being evil...

.. and reading about an old DOS trick with an edited prompt, I came up with this way of annoying the hell out of a user.

/User is presented with:

Enter an 11-digit prime number to continue.

If the number is the wrong length, they get:

That number was not 11-digits.

If the number isn't a prime number:

That number isn't prime. (followed by the original prompt)

If the number is prime, it checks to see if they've used it before.

If they have. They get "Not that one". And back to the original prompt.

If they haven't used it before, there's a 50% chance it'll give them "not that one" anyway and add it to the list of "used" prime numbers.

Any bets on how long before the user wants to kill the system?