Debug Help! It's driving me nuts

1. This works (browsing), click on URL and add product to cart

Home > Supplies > Printer, Laser > Hewlett-Packard > HP LaserJet 4000 Series


Add a product to the cart via the a yellow button “Add to Cart”. This works as expected.

2. This works as well (searching), click on URL then add a product to cart

Searching for “HP LaserJet 4000 Series”


This works as expected.

3. Now try this link (searching), and add an item to the cart

Searching for “HEWLETT-PACKARD Black Print Cartridge”


It does not work. It does not add any item to the cart, and it turns the cart sidebox to the full cart, and the AJAX hangs.