If you disable a product and then try to access it on the front end of the site, this leads to a white page with 500 Server Error in the page title. Alternatively you may see the full 500 Internal Server Error page instead of a white page. This is a bug in Magento and can be fixed by editing this file: lib/Varien/Autoload.php

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staticpublicfunction registerScope($code)

and change this text

@include COMPILER_INCLUDE_PATH

to this

@include_once COMPILER_INCLUDE_PATH

Source: https://stackoverflow.com/questions/18595658/magento-disabled-product-shows-500-internal-server-error