This issue is caused by the donation form's configuration. If a donation form uses a payment page that is set to keep anonymous carts for 1+ days, this can happen if a user went through the steps of adding a donation but did not finish checking out at the time so the anonymous cart was kept cached in the browser. This can be prevented by using one of two methods:

1) Using proceed directly to payment on the donation form which doesn’t keep anonymous carts and sends the user directly to the Blackbaud Secure Payment Page. This change is made on the donation form itself.
2) Setting Payment pages to not keep anonymous carts for any amount of time. This change is made on the payment 2.0 part.