When you are on other thank you page (after checkout is complete) and then you go drink so tee for an 2 hours or so, then when you return session expiration happens or not happens. Anyway session is dead by then. On that thank you template, when order contents is printed it's ID is retrieved from session via "front_order_id" variable. When session is gone, then ID is gone too, so instead of products in order we are getting all possible products from database which most likely will end up with fatal error about too much memory being used by the script.
I propose to use FAKE_ORDER_ID constant as order id in case, when ID in session is lost. This will give us empty order and no errors.