Best way to handle missing url parameters

[eluser]jplanet[/eluser]
I found a hack to the CI core that someone posted for handling required parameters, but of course I would like to make sure that the client can keep CI updated in the future. What is the cleanest way to 404 if a user messes with the url and removes required parameters? naturally, I would like to find something cleaner than configuring routes for each and every possible combination and/or writing code in every single function in all controllers...