Why do I see errors related to Salesforce State and Country picklists on confirmation pages?

If you run into the following error message on the confirmation page for Events, Petitions, Actions, Forms or Shop, it is because the State and Country picklist options Salesforce introduced are supported only by the Donations app at the current time. If these are enabled within your Salesforce instance and you use any app write to Salesforce other than Donations, it will cause this error and not allow records to be created.

Error Message:

Salesforce Error: There's a problem with this country, even though it may appear correct. Please select a country from the list of valid countries.: Country

Solution:

Follow instructions provided by Salesforce to disable State and Country picklists.