Steps to regenerate
Say, I have an upload complete cart form with some validation in the UploadCompleteCartButton1_Click(). If the validation fails and I call CancelSubmission() it works fine. But if I then click another button on the form (for example a button to update the quantity for an item) it then submits the form to paypal where I get a"no items in the cart"error. The only way I have found around this is to have theCancelSubmission() in every button click event handler.