Merchant address re-use? I understand that one way a merchant would likely use Bitcoin is to create a unique receipt address for every customer, so when a given address receives payment, the merchant can know exactly which customer sent in payment.

Ideally, these addresses would get used one time, and then thrown away.
But the world isn't an ideal place, and customers sometimes (frequently) make mistakes.   

To prevent Bitcoin loss, would a merchant need to keep every address (and associated private key) forever?  Doesn't that become burdensome?
To prevent Bitcoin loss, would a merchant need to keep every address (and associated private key) forever?


Yes.


  Doesn't that become burdensome?


Not really. First of all, private keys and addresses are fairly small, so they don't take up that much space. Putting them into a database can make access very quick and easy so that's not much of a problem. Furthermore, merchants already have to keep the private keys around in order to spend the coins from those addresses later. It is uneconomical for them to immediately forward the funds to some other storage address as they will incur transaction fees for doing so.