One of my customers modified the mapping between Account and Contact and from that point forward, the received the following error message ( DevErrors are turned on ) when attempting to add a new Contact to an existing Account:

Now that was rather confusing because it only happened if you had an Account open, selected Contacts, then clicked the New button.  Clicking the New button from a Contacts View did not generate an error.

Solution

After Googling around a while I ran into this post in the CRM  newsgroups.  Basically, you needed to delete the following mappings:

Source Schema name: "Address1_addressid"
Target Schema name: "Address1_addressid"

Source Schema name: "Address2_addressid"
Target Schema name: "Address2_addressid"

Thanks to Aami for discovering and posting the solution.