Preparations

1. in Admin Console
   1. go to Website & Content → Structure & Data → Products section
   2. on Sections tab add Sub Products category
   3. go to Website & Content → Structure & Data → Products → Sub Products section
   4. on Products tab add new product
2. in IDE:
   1. open platform/designs/default_design.des template in advanced theme for editing
   2. right above <inp2:m_RenderElement name="content" default_element="cms_content"/> line add [<inp2:m_FormAction/>]<br/> line

Part 1

• in Admin Console:
  1. go to Configuration → Website → Advanced section
  2. check "Enable SEO-friendly URLs mode (MOD-REWRITE)" checkbox and save changes
• on Front-End:
  1. open url of non-existing page (e.g. /tra-la-la)
  2. confirm, that 404 page is shown and HTTP response code is also 404
  3. go to Products → Sub Products section in top menu
  4. confirm, that products created before are shown
  5. add (must be in mod-rewrite mode) ?page=10 to the url (list must not have 10th page)
  6. confirm, that 404 page is shown and HTTP response code is also 404

Part 2

• in Admin Console:
  1. go to Configuration → Website → Advanced section
  2. uncheck "Enable SEO-friendly URLs mode (MOD-REWRITE)" checkbox and save changes
• on Front-End:
  1. go to Products → Sub Products category using top menu
  2. click on the product name (a link) to open it's detail page
  3. note place above navigation bar, where added template line result is now showing
  4. confirm, that %3Ap-1---- part from URL is show above navigation bar (the 1 is ID of created product)
  5. in the URL replace ID of product with 9999 (non-existing product id)
  6. confirm, that %3Ap-9999---- part from URL isn't show above navigation bar (the 9999 is ID of non-existing product)
  7. use Back button of web browser to see correct product detail page again
  8. press Buy Now button to add product to cart
  9. confirm, that redirect to Shopping Cart page happened and %3Ap-1---- is present in the URL (the 1 is ID of added product)
  10. locate part in URL, that looks like %3Am141- (the 141 is ID of category)
  11. change 141 (or whatever number is shown in your case) to 0
  12. change shop_cart in the URL into shopcart
  13. confirm, that %3Ap-1---- part from URL isn't show above navigation bar (the 1 is ID of product from URL)

Part 3

• in IDE:
  1. disable debug mode in /system/debug.php file
  2. in any kMainTagProcessor class add AlwaysNotFound tag, that would call $this->Application->UrlManager->show404();
  3. replace content of error_notfound template in theme with 404 page
  4. add /themes/advanced/test.tpl template with <inp2:m_include template="platform/designs/default_design.des" pass_params="1"/> content
  5. replace contents of /themes/advanced/platform/designs/default_design.des.tpl template with following:

<inp2:m_DefineElement name="elem_test">
	<inp2:m_AlwaysNotFound/>
</inp2:m_DefineElement>

<inp2:m_RenderElement name="elem_test" />

• in Admin Console:
  1. go to Configuration → Website → Advanced section
  2. ensure, that Enable HTML GZIP compression checkbox is checked (save changes if needed)
  3. go to Configuration → Website → Themes section
  4. press Rescan Themes button on toolbar
• in Web Browser:
  1. go to http://www.website.tld/test.html page
  2. confirm, that Page Not Found page is displayed, but url stays the same