session issues?

recently i have had 2 customers have some issues with i think session data.
orders have come through where the product price is 0 for both customers. so i thought maybe this error occured before the cart when it was pulling up the product record. however it gets the product name and product number correct.
both orders that had error, later were placed without error.


and we have had an order where the billing and shipping info was blank.
it did not save in the database or generate an order# but it sent an email.
i was thinking this info was lost session info. but why would it lose this info and not other info?

does the cart info last longer as session data than checkout data?
any idea how an item can be added to cart as $0 only to be added later at its correct price?

any helpful guidance of where to look is appreciated. this is 2 separate clients on 2 different hosts and it has happened to both in the last few days. one cart is much older than the other. so weird.