Exact error
Text and code, and where it appears: browser, SP log, IdP log.
Protocol artifact
Decoded SAMLResponse or decoded ID token claims, with signatures removed.
Expected values
entityID, ACS URL, client_id, issuer, redirect_uri as configured.
Recent changes
Anything touched in the last few days on either side.
Never paste passwords, private keys, client secrets, session cookies, or raw access/ID tokens.