diff --git a/.github/scripts/fork_sync_digest.py b/.github/scripts/fork_sync_digest.py
index 3820c4e..cc8affb 100644
--- a/.github/scripts/fork_sync_digest.py
+++ b/.github/scripts/fork_sync_digest.py
@@ -89,7 +89,7 @@ def load_forks(path: str | Path = ".github/forks.yml") -> list[dict]:
     return out
 
 
-def sync_branch(repo: str, branch: str) -> dict:
+def sync_branch(repo: str, branch: str, upstream_org: str | None = None) -> dict:
     status, body = gh("POST", f"/repos/{repo}/merge-upstream", {"branch": branch})
     msg = body.get("message", "")
     # "Branch n/a" responses we expect on forks that haven't cut a 20.0
@@ -100,14 +100,52 @@ def sync_branch(repo: str, branch: str) -> dict:
     skipped = (status == 422 and "does not exist" in msg.lower()) or (
         status == 404 and "branch not found" in msg.lower()
     )
-    return {
+    result = {
         "repo": repo,
         "branch": branch,
         "status": status,
         "message": msg,
         "merge_type": body.get("merge_type"),
         "skipped": skipped,
+        "ahead_by": None,
+        "behind_by": None,
+        "diverged": False,
     }
+    # Post-sync divergence check. merge-upstream silently produces a
+    # merge commit (instead of fast-forwarding) when the fork has local
+    # commits the upstream doesn't have — returning success either way.
+    # Without this check, accidental pushes to a series-named branch on
+    # the fork (e.g. someone "Merge pull request"-ing a feature branch
+    # into 18.0) accumulate undetected and the daily sync keeps adding
+    # merge commits on top. Compare against upstream to surface drift.
+    if upstream_org and not skipped and status < 400:
+        ahead, behind = _compare_with_upstream(repo, upstream_org, branch)
+        result["ahead_by"] = ahead
+        result["behind_by"] = behind
+        result["diverged"] = bool(ahead and ahead > 0)
+    return result
+
+
+def _compare_with_upstream(
+    fork_repo: str, upstream_org: str, branch: str
+) -> tuple[int | None, int | None]:
+    """Return (ahead_by, behind_by) for fork branch vs upstream branch.
+
+    ahead_by > 0 means the fork has commits the upstream doesn't —
+    almost always a mistake on a series-named tracking branch.
+    Returns (None, None) on any API error so a flaky compare doesn't
+    mask a successful sync.
+    """
+    fork_owner = fork_repo.split("/", 1)[0]
+    repo_name = fork_repo.split("/", 1)[1]
+    path = (
+        f"/repos/{upstream_org}/{repo_name}/compare/"
+        f"{upstream_org}:{branch}...{fork_owner}:{branch}"
+    )
+    status, body = gh("GET", path)
+    if status != 200:
+        return None, None
+    return body.get("ahead_by"), body.get("behind_by")
 
 
 def list_recent_mig_prs(
@@ -141,11 +179,13 @@ def main() -> int:
     sync_results: list[dict] = []
     for f in forks:
         for branch in f.get("branches", []):
-            res = sync_branch(f["repo"], branch)
+            res = sync_branch(f["repo"], branch, upstream_org=f.get("upstream_org"))
             sync_results.append(res)
             tag = "SKIP" if res["skipped"] else (
                 res.get("merge_type") or str(res["status"])
             )
+            if res.get("diverged"):
+                tag = f"{tag} DIVERGED+{res['ahead_by']}"
             print(f"  {res['repo']}@{branch:>10}  -> {tag}", file=sys.stderr)
     Path("sync-results.json").write_text(json.dumps(sync_results, indent=2))
 
diff --git a/.github/scripts/render_digest.py b/.github/scripts/render_digest.py
index 5c4bb0e..90fe8e3 100644
--- a/.github/scripts/render_digest.py
+++ b/.github/scripts/render_digest.py
@@ -44,6 +44,14 @@ def render(
         if r.get("merge_type") == "none" and r["status"] < 400
     ]
     sync_skipped = [r for r in sync_results if r["skipped"]]
+    # Divergence = fork branch has commits upstream doesn't. merge-upstream
+    # returns success and produces a merge commit; the digest needs to
+    # surface this separately so it gets fixed (force-reset the branch)
+    # instead of accumulating forever.
+    sync_diverged = [
+        r for r in sync_results
+        if r.get("diverged") and not r["skipped"]
+    ]
 
     dist_fail = [r for r in distribution if r["action"] == "failed"]
     dist_created = [r for r in distribution if r["action"] == "created"]
@@ -58,7 +66,12 @@ def render(
         f"{len(sync_nochange)} already current · "
         f"{len(sync_skipped)} skipped (branch n/a) · "
         f'<span style="color:{"#c00" if sync_fail else "#0a0"}">'
-        f"{len(sync_fail)} failed</span></p>",
+        f"{len(sync_fail)} failed</span>"
+        + (
+            f' · <span style="color:#c80">{len(sync_diverged)} diverged</span>'
+            if sync_diverged else ""
+        )
+        + "</p>",
     ]
     if distribution:
         lines.append(
@@ -79,6 +92,24 @@ def render(
             )
         lines.append("</ul>")
 
+    if sync_diverged:
+        lines.append(
+            '<h3 style="color:#c80">⚠️ Diverged from upstream</h3>'
+            "<p>These fork branches have commits the upstream doesn't — "
+            "merge-upstream silently produced a merge commit instead of "
+            "fast-forwarding. Reset the branch to upstream HEAD "
+            "(after backing it up).</p><ul>"
+        )
+        for r in sync_diverged:
+            ahead = r.get("ahead_by") or 0
+            behind = r.get("behind_by") or 0
+            lines.append(
+                f"<li><code>{html.escape(r['repo'])}</code> "
+                f"({html.escape(r['branch'])}): "
+                f"<b>{ahead} ahead</b>, {behind} behind</li>"
+            )
+        lines.append("</ul>")
+
     if dist_fail:
         lines.append('<h3 style="color:#c00">⚠️ Distributor failures</h3><ul>')
         for r in dist_fail:
@@ -132,6 +163,8 @@ def render(
         subject_parts.append(f"⚠️ {len(sync_fail)} sync fail")
     if dist_fail:
         subject_parts.append(f"⚠️ {len(dist_fail)} dist fail")
+    if sync_diverged:
+        subject_parts.append(f"⚠️ {len(sync_diverged)} diverged")
     if total_migs:
         subject_parts.append(f"{total_migs} MIG")
     subject = " — ".join(subject_parts)
diff --git a/.github/scripts/tests/test_render_digest.py b/.github/scripts/tests/test_render_digest.py
index a3650db..aa52dc8 100644
--- a/.github/scripts/tests/test_render_digest.py
+++ b/.github/scripts/tests/test_render_digest.py
@@ -107,6 +107,34 @@ def test_both_sync_and_dist_failures_in_subject():
     assert exit_marker == "2"
 
 
+def test_diverged_branch_warning_in_subject_and_body():
+    subj, body, exit_marker = render_digest.render(
+        sync_results=[
+            _sync(200, "fast-forward"),
+            {
+                "repo": "ledoent/social",
+                "branch": "18.0",
+                "status": 200,
+                "message": "",
+                "merge_type": "merge",
+                "skipped": False,
+                "ahead_by": 30,
+                "behind_by": 0,
+                "diverged": True,
+            },
+        ],
+        mig_buckets={},
+        distribution=[],
+    )
+    assert "⚠️ 1 diverged" in subj
+    # Divergence is a warning, not a failure — exit_marker should still be 0
+    # so the workflow doesn't conflate "needs manual fix" with "scripts broke".
+    assert exit_marker == "0"
+    assert "Diverged from upstream" in body
+    assert "ledoent/social" in body
+    assert "30 ahead" in body
+
+
 def test_html_escaping_in_failure_message():
     # Defensive: an API error message containing < > & shouldn't break
     # the rendered HTML (or open an XSS hole if anyone ever pipes the
diff --git a/.github/scripts/tests/test_sync_branch_state.py b/.github/scripts/tests/test_sync_branch_state.py
index 4f9efc9..0132bac 100644
--- a/.github/scripts/tests/test_sync_branch_state.py
+++ b/.github/scripts/tests/test_sync_branch_state.py
@@ -56,3 +56,84 @@ def test_conflict_409_is_not_skipped(monkeypatch):
     r = fork_sync_digest.sync_branch("ledoent/x", "18.0")
     assert r["skipped"] is False
     assert r["status"] == 409
+
+
+def _patch_gh_sequence(monkeypatch, *returns):
+    """Make gh() return each tuple in turn (merge-upstream, then compare)."""
+    it = iter(returns)
+    monkeypatch.setattr(fork_sync_digest, "gh", lambda *a, **kw: next(it))
+
+
+def test_diverged_branch_is_flagged_when_upstream_org_provided(monkeypatch):
+    # merge-upstream succeeds with a merge commit; compare reports 30
+    # local commits ahead. This is the case that motivated the check —
+    # ledoent/social:18.0 in May 2026 silently accumulated drift because
+    # merge-upstream returns success either way.
+    _patch_gh_sequence(
+        monkeypatch,
+        (200, {"merge_type": "merge"}),
+        (200, {"ahead_by": 30, "behind_by": 0}),
+    )
+    r = fork_sync_digest.sync_branch("ledoent/social", "18.0", upstream_org="OCA")
+    assert r["diverged"] is True
+    assert r["ahead_by"] == 30
+    assert r["behind_by"] == 0
+
+
+def test_clean_fast_forward_is_not_diverged(monkeypatch):
+    _patch_gh_sequence(
+        monkeypatch,
+        (200, {"merge_type": "fast-forward"}),
+        (200, {"ahead_by": 0, "behind_by": 0}),
+    )
+    r = fork_sync_digest.sync_branch("ledoent/x", "18.0", upstream_org="OCA")
+    assert r["diverged"] is False
+    assert r["ahead_by"] == 0
+
+
+def test_no_upstream_org_skips_divergence_check(monkeypatch):
+    # Non-OCA forks (upstream_org: null) get no compare call. Verified by
+    # patching gh to fail loudly if called more than once.
+    calls = []
+
+    def fake_gh(method, path, body=None):
+        calls.append((method, path))
+        return 200, {"merge_type": "fast-forward"}
+
+    monkeypatch.setattr(fork_sync_digest, "gh", fake_gh)
+    r = fork_sync_digest.sync_branch("ledoent/x", "master")
+    assert len(calls) == 1
+    assert r["diverged"] is False
+    assert r["ahead_by"] is None
+
+
+def test_compare_failure_does_not_mask_successful_sync(monkeypatch):
+    # If the compare endpoint flakes, the sync result still records the
+    # merge-upstream success. ahead_by stays None so the digest knows we
+    # don't actually know — better than a false "clean" classification.
+    _patch_gh_sequence(
+        monkeypatch,
+        (200, {"merge_type": "fast-forward"}),
+        (500, {"message": "Internal Server Error"}),
+    )
+    r = fork_sync_digest.sync_branch("ledoent/x", "18.0", upstream_org="OCA")
+    assert r["status"] == 200
+    assert r["merge_type"] == "fast-forward"
+    assert r["diverged"] is False
+    assert r["ahead_by"] is None
+
+
+def test_failed_sync_skips_compare(monkeypatch):
+    # No point burning a compare call if merge-upstream itself failed —
+    # the failure already triggers a digest alarm.
+    calls = []
+
+    def fake_gh(method, path, body=None):
+        calls.append((method, path))
+        return 403, {"message": "Resource not accessible by personal access token"}
+
+    monkeypatch.setattr(fork_sync_digest, "gh", fake_gh)
+    r = fork_sync_digest.sync_branch("ledoent/x", "18.0", upstream_org="OCA")
+    assert len(calls) == 1
+    assert r["status"] == 403
+    assert r["diverged"] is False