HTTP Status 409 - Conflict

PUT /api/documents/doc-123 HTTP/1.1
Host: api.example.com
If-Match: "v1-abc"
Content-Type: application/json

{"content": "Updated text"}

HTTP/1.1 409 Conflict
ETag: "v2-def"
Content-Type: application/json

{
  "error": "version_conflict",
  "message": "Document was modified by another user",
  "expected_version": "v1-abc",
  "current_version": "v2-def",
  "action": "Fetch latest version and retry"
}

POST /api/users HTTP/1.1
Host: api.example.com
Content-Type: application/json

{"username": "alice", "email": "alice@example.com"}

HTTP/1.1 409 Conflict
Content-Type: application/json

{
  "error": "duplicate_resource",
  "message": "Username 'alice' already exists",
  "conflicting_field": "username",
  "suggestion": "Choose a different username"
}

POST /api/orders/789/cancel HTTP/1.1
Host: api.example.com

HTTP/1.1 409 Conflict
Content-Type: application/json

{
  "error": "invalid_state_transition",
  "message": "Cannot cancel order in 'shipped' state",
  "current_state": "shipped",
  "allowed_transitions": ["return", "complete"]
}