diff --git a/packages/marshal/package.json b/packages/marshal/package.json
index 21986b2982..e0eff7ed4b 100644
--- a/packages/marshal/package.json
+++ b/packages/marshal/package.json
@@ -41,6 +41,7 @@
   },
   "homepage": "https://github.com/endojs/endo#readme",
   "dependencies": {
+    "@endo/common": "^1.0.2",
     "@endo/errors": "^1.0.2",
     "@endo/eventual-send": "^1.1.0",
     "@endo/nat": "^5.0.2",
diff --git a/packages/marshal/src/marshal-justin.js b/packages/marshal/src/marshal-justin.js
index 240328774f..69bd2d7ed6 100644
--- a/packages/marshal/src/marshal-justin.js
+++ b/packages/marshal/src/marshal-justin.js
@@ -390,6 +390,10 @@ const decodeToJustin = (encoding, shouldIndent = false, slots = []) => {
 
         case 'error': {
           const { name, message } = rawTree;
+          // TODO cause, errors, AggregateError
+          // See https://github.com/endojs/endo/pull/2052
+          name !== `AggregateError` ||
+            Fail`AggregateError not yet implemented in marshal-justin`;
           return out.next(`${name}(${quote(message)})`);
         }
 
diff --git a/packages/marshal/src/marshal.js b/packages/marshal/src/marshal.js
index fe2b419e3b..edc8bb1d2c 100644
--- a/packages/marshal/src/marshal.js
+++ b/packages/marshal/src/marshal.js
@@ -9,6 +9,7 @@ import {
 } from '@endo/pass-style';
 
 import { X, Fail, q, makeError, annotateError } from '@endo/errors';
+import { objectMap } from '@endo/common/object-map.js';
 import {
   QCLASS,
   makeEncodeToCapData,
@@ -260,11 +261,11 @@ export const makeMarshal = (
      */
     const decodeErrorCommon = (errData, decodeRecur) => {
       const { errorId = undefined, message, name, ...rest } = errData;
-      ownKeys(rest).length === 0 ||
-        Fail`unexpected encoded error properties ${q(ownKeys(rest))}`;
-      // TODO Must decode `cause` and `errors` properties
-      // capData does not transform strings. The calls to `decodeRecur`
-      // are for reuse by other encodings that do, such as smallcaps.
+      // TODO Must decode `cause` and `errors` properties.
+      // See https://github.com/endojs/endo/pull/2052
+      // capData does not transform strings. The immediately following calls
+      // to `decodeRecur` are for reuse by other encodings that do,
+      // such as smallcaps.
       const dName = decodeRecur(name);
       const dMessage = decodeRecur(message);
       const dErrorId = errorId && decodeRecur(errorId);
@@ -279,6 +280,14 @@ export const makeMarshal = (
           ? `Remote${EC.name}`
           : `Remote${EC.name}(${dErrorId})`;
       const error = makeError(dMessage, EC, { errorName });
+      if (ownKeys(rest).length >= 1) {
+        // Note that this does not decodeRecur rest's property names.
+        // This would be inconsistent with smallcaps' expected handling,
+        // but is fine here since it is only used for `annotateError`,
+        // which is for diagnostic info that is otherwise unobservable.
+        const extras = objectMap(rest, decodeRecur);
+        annotateError(error, X`extra marshalled properties ${extras}`);
+      }
       return harden(error);
     };
 
diff --git a/packages/marshal/test/test-marshal-capdata.js b/packages/marshal/test/test-marshal-capdata.js
index a828ac7e2f..38ba096da1 100644
--- a/packages/marshal/test/test-marshal-capdata.js
+++ b/packages/marshal/test/test-marshal-capdata.js
@@ -4,7 +4,13 @@ import { test } from './prepare-test-env-ava.js';
 import { passStyleOf, makeTagged, Far } from '@endo/pass-style';
 import { makeMarshal } from '../src/marshal.js';
 
-const { freeze, isFrozen, create, prototype: objectPrototype } = Object;
+const {
+  freeze,
+  isFrozen,
+  create,
+  prototype: objectPrototype,
+  getPrototypeOf,
+} = Object;
 
 const harden = /** @type {import('ses').Harden & { isFake?: boolean }} */ (
   // eslint-disable-next-line no-undef
@@ -295,6 +301,41 @@ test('unserialize errors', t => {
   t.is(em3.message, 'msg3');
 });
 
+test('unserialize extended errors', t => {
+  const { unserialize } = makeTestMarshal();
+  const uns = body => unserialize({ body, slots: [] });
+
+  // TODO cause, errors, and AggregateError will eventually be recognized.
+  // See https://github.com/endojs/endo/pull/2042
+
+  const refErr = uns(
+    '{"@qclass":"error","message":"msg","name":"ReferenceError","extraProp":"foo","cause":"bar","errors":["zip","zap"]}',
+  );
+  t.is(getPrototypeOf(refErr), ReferenceError.prototype); // direct instance of
+  t.false('extraProp' in refErr);
+  t.false('cause' in refErr);
+  t.false('errors' in refErr);
+  console.log('error with extra prop', refErr);
+
+  const aggErr = uns(
+    '{"@qclass":"error","message":"msg","name":"AggregateError","extraProp":"foo","cause":"bar","errors":["zip","zap"]}',
+  );
+  t.is(getPrototypeOf(aggErr), Error.prototype); // direct instance of
+  t.false('extraProp' in aggErr);
+  t.false('cause' in aggErr);
+  t.false('errors' in aggErr);
+  console.log('error with extra prop', aggErr);
+
+  const unkErr = uns(
+    '{"@qclass":"error","message":"msg","name":"UnknownError","extraProp":"foo","cause":"bar","errors":["zip","zap"]}',
+  );
+  t.is(getPrototypeOf(unkErr), Error.prototype); // direct instance of
+  t.false('extraProp' in unkErr);
+  t.false('cause' in unkErr);
+  t.false('errors' in unkErr);
+  console.log('error with extra prop', unkErr);
+});
+
 test('passStyleOf null is "null"', t => {
   t.assert(passStyleOf(null), 'null');
 });
diff --git a/packages/marshal/test/test-marshal-smallcaps.js b/packages/marshal/test/test-marshal-smallcaps.js
index d413e87b68..714e155596 100644
--- a/packages/marshal/test/test-marshal-smallcaps.js
+++ b/packages/marshal/test/test-marshal-smallcaps.js
@@ -6,7 +6,13 @@ import { makeMarshal } from '../src/marshal.js';
 
 import { roundTripPairs } from './test-marshal-capdata.js';
 
-const { freeze, isFrozen, create, prototype: objectPrototype } = Object;
+const {
+  freeze,
+  isFrozen,
+  create,
+  prototype: objectPrototype,
+  getPrototypeOf,
+} = Object;
 
 const harden = /** @type {import('ses').Harden & { isFake?: boolean }} */ (
   // eslint-disable-next-line no-undef
@@ -153,6 +159,41 @@ test('smallcaps unserialize errors', t => {
   t.is(em3.message, 'msg3');
 });
 
+test('smallcaps unserialize extended errors', t => {
+  const { unserialize } = makeTestMarshal();
+  const uns = body => unserialize({ body, slots: [] });
+
+  // TODO cause, errors, and AggregateError will eventually be recognized.
+  // See https://github.com/endojs/endo/pull/2042
+
+  const refErr = uns(
+    '#{"#error":"msg","name":"ReferenceError","extraProp":"foo","cause":"bar","errors":["zip","zap"]}',
+  );
+  t.is(getPrototypeOf(refErr), ReferenceError.prototype); // direct instance of
+  t.false('extraProp' in refErr);
+  t.false('cause' in refErr);
+  t.false('errors' in refErr);
+  console.log('error with extra prop', refErr);
+
+  const aggErr = uns(
+    '#{"#error":"msg","name":"AggregateError","extraProp":"foo","cause":"bar","errors":["zip","zap"]}',
+  );
+  t.is(getPrototypeOf(aggErr), Error.prototype); // direct instance of
+  t.false('extraProp' in aggErr);
+  t.false('cause' in aggErr);
+  t.false('errors' in aggErr);
+  console.log('error with extra prop', aggErr);
+
+  const unkErr = uns(
+    '#{"#error":"msg","name":"UnknownError","extraProp":"foo","cause":"bar","errors":["zip","zap"]}',
+  );
+  t.is(getPrototypeOf(unkErr), Error.prototype); // direct instance of
+  t.false('extraProp' in unkErr);
+  t.false('cause' in unkErr);
+  t.false('errors' in unkErr);
+  console.log('error with extra prop', unkErr);
+});
+
 test('smallcaps mal-formed @qclass', t => {
   const { unserialize } = makeTestMarshal();
   const uns = body => unserialize({ body, slots: [] });