It seems that a couple of customers for one of my ZenCart vendors (using 1.2.7) is having a strange problem. I have a theory that I want to run by everyone - please lend your feedback!

Situation is:

A certain coffee product was recently changed to "out of stock", and with several tests the merchant and we have verified it is not possible to order it from the site. However, a couple of shoppers have reported that this coffee is showing up in their cart and they are not able to delete it at all. Even if they log out and try to order from the beginning, that silly product continues to appear in their cart.

My theory:

They previously added this product to their cart before it was out of stock and it was saved in the cart when they returned. Since it is now out of stock, they are unable to delete it. (Maybe if it was in stock, they could successfully delete it.)

Or) Something is cached in their browser which is not allowing the cart to update?

Do you think I could be right? Or could this be some sort of problem with this particular product?

Any advice is appreciated.