Create a test.html with the following content:
The form action can also just as well be demo.cs-cart.com.
Once submitted, the results are unaccounted for. A variable posted to a page that is unexpected should not have any effect on the site itself.
Rather it does not fail gracefully displaying the page specified in the address bar. There are errors and it shows either a 404 for the home page, or a blank page missing elements.
Simply put, It breaks the site.