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The 14 things to be sure of

If you remember nothing else, remember these.

1	SOAP is a protocol with a strict contract; REST is an architectural style. That's the whole difference in one line.
2	Envelope → optional Header (security, routing) → mandatory Body. Errors go in a <Fault> inside the Body.
3	A SOAP fault still returns HTTP 500 — the status code tells you nothing; read the fault.
4	1.1 vs 1.2: different namespace, application/soap+xml instead of text/xml, and Code/Reason instead of faultcode/faultstring.
5	WSDL = types · message · portType · binding · service. The client needs the binding and the service address.
6	SOAP is not encrypted by itself. You get privacy from HTTPS (transport) or WS-Security (message).
7	Transport security protects point to point; message security survives intermediaries. That's when you need WS-Security.
8	Interop problems are nearly always namespaces, SOAPAction, or a missing security header — in that order.
9	WCF's ABC: Address (where), Binding (how), Contract (what).
10	basicHttpBinding = interop · wsHttpBinding = WS-* features · netTcpBinding = fast, .NET-to-.NET only · webHttpBinding = REST, not SOAP.
11	Throw FaultException<T>, never a raw .NET exception — a plain exception leaks nothing useful and faults the channel.
12	A faulted proxy must be Abort()ed and recreated. Reusing it throws CommunicationObjectFaultedException.
13	.NET Core can consume SOAP (dotnet-svcutil, async-only proxies, binding configured in code) but cannot host WCF — use CoreWCF or SoapCore.
14	The modern answer to any legacy-SOAP question: wrap it in a .NET Core REST facade with DTOs, caching, Polly and JWT, so the frontend only sees JSON.

Debugging order that actually works

1	Can you fetch ?wsdl at all? → network, TLS, auth.
2	Does SoapUI succeed with its generated request? → the service is fine; it's your client.
3	Diff SoapUI's envelope against yours → namespace, SOAPAction, headers.
4	Log the full request and response envelopes, not just the exception message.

Words that signal seniority

contract-first · interoperability · WS-I Basic Profile · document/literal · message vs transport security · typed fault · faulted channel · facade · anti-corruption layer · legacy seam · MTOM · binding · trade-off