Looking at secstate's post reminds me of one of the neater things (at least to me) in FHIR models, that a patient can also represent an animal. I have only worked with HIEs and EHRs for humans so I don't know what veterinarians' EHRs may be using. Do they use race and ethnicity fields for animal type and breed?

Anyway, my only other IDC related story is that of going from ICD-9 to ICD-10, when decimals came into play. We had several projects where we had to go back and do global replaces on ICD-9 codes to play nicely with the ICD-10 codes.