FHIR Extensibility Page
Also see discussion on page here.
Extension applied at invalid location
When extensions are defined, we identify what resource path (or possibly pattern) they apply to. What happens if an extension is received that references an id on a location that isn't actually a legal extension for that element? Presume that's an error. Are applications expected to check this? (At least for extensions they support?) For that matter, do we need to make any statements about what systems can or should do if they receive an instance that contains an error? Are they allowed to process it?