I started regenerating thumbnails before I found this thread. Would there still be a benefit to me using your directions, or did I create a hopeless mess by already regenerating?
We are having the same problem. A customer emailed us last night, having trouble placing an order, because she could not change the quantities in the cart and recalculate. When she tried to check out, it reverted back to one of each item.
We are very concerned that we are losing sales because of this problem. It makes it very hard for a customer to complete an order.