I just tested the 301 update for the for product detail and the following two different errors occured on two different sites. I didn't include the pp_XX.X pages as I don't use PayPal
Site 1: The product details were captured and the transaction went through however rather than reverting back to the checkout_success.php page it reverted back to the cart.php page. All emails were also sent.
Site 2: Again the transaction was processed, admin updated and emails sent however in this case while it also reverted back to the cart.php page instead of the checkout_success.php page it also displayed that the inventory was updated with what was in stock. So two errors on site 2, reverting to the cart.php while the transaction went through, emails sent etc and also displaying the updated stock error. Screen shot attached. [[[When I look at managing inventory it still shows up as one rather than zero even after it was sold although when I go to update product details the inventory system works properly here. Also on initial sale the email that is sent contains colour and size, however if I send the receipt again in order admin then the size and colour detail is missing]]]