Magento – Magento product url gives 404 page

I had a problem that responded well to commenting out the line you suggest too.:

Mage::dispatchEvent('catalog_controller_product_view', array('product' => $product));

so this got me thinking to look for anything using that event, so i did a grep on app/code for catalog_controller_product_view

not, much, considering the first one is just that which has the line we commented. then i see something in Reports, and that jogs my memory... i didnt think it was necessary but i had emptied the report_* tables during a database merge, this must have been it,

so after restoring those tables from a backup its working again even with the line uncommented... !

404 issues are quite common when migrating Magento installations between servers. As per this post on SO please see the following answer

Magento Admin 404

You need to go your server directly and do this via SSH/ FTP

You have to delete the following file:

app/etc/use_cache.ser

If you get an error after that like:

Notice: Undefined index: 0 in /srv/www/vhosts/javra.com/htdocs/munchad2/app/code/core/Mage/Core/Model/Mysql4/Config.php on line 92

Then go to your Database Management:

• Open PhpMyAdmin
• Go to your database
• Click SQL
• Run the following SQL Query

_

SET FOREIGN_KEY_CHECKS=0;
UPDATE `core_store` SET store_id = 0 WHERE code='admin';
UPDATE `core_store_group` SET group_id = 0 WHERE name='Default';
UPDATE `core_website` SET website_id = 0 WHERE code='admin';
UPDATE `customer_group` SET customer_group_id = 0 WHERE customer_group_code='NOT LOGGED IN';
SET FOREIGN_KEY_CHECKS=1;

Definitely backup the file & your database prior to running these changes.

Alternately, double check your .htaccess files which are also sometimes the bane of the issue.