I successfully replicated this problem using the API tester:   I use the "Retrieve a contact with a specified Contact ID" endpoint: https://api.constantcontact.com/v2/contacts/xxxxx?api_key=yyyyyy   In most cases, I can copy the result body and paste it in (unmodified!) as the request body for the endpoint "Modify a contact with specific Contact ID"   https://api.constantcontact.com/v2/contacts/xxxxx?action_by=ACTION_BY_OWNER&api_key=yyyyyy   In about 90% of test cases, this works successfully.  In the remaining 10%, I get: Response StatusSelect content 409 Conflict Response HeadersSelect content Cache-Control: private, no-cache, no-store, max-age=0, must-revalidate, no-cache="Set-Cookie" Content-Type: application/json Date: Thu, 17 Mar 2016 01:09:53 GMT Pragma: no-cache Server: Apache Vary: Accept-Encoding,User-Agent X-Mashery-Responder: prod-j-worker-us-east-1e-114.mashery.com X-Powered-By: Content-Length: 130 Connection: keep-alive Response BodySelect content [{     "error_key": "http.status.conflict",     "error_message": "There was a conflict between the supplied data and the existing resource." }]  Is there a reason why this would be?  My actual use case is more complex, but I created this simple test case to eliminate any other variables.   Jon ... View more