diff --git a/ghstack/submit.py b/ghstack/submit.py
index 1e801af..e3cddeb 100644
--- a/ghstack/submit.py
+++ b/ghstack/submit.py
@@ -378,7 +378,7 @@ def run(self) -> List[DiffMeta]:
         ]
         self.push_updates(diffs_to_submit)
         if new_head := rebase_index.get(
-            GitCommitHash(self.sh.git("rev-parse", "HEAD"))
+            old_head := GitCommitHash(self.sh.git("rev-parse", "HEAD"))
         ):
             self.sh.git("reset", "--soft", new_head)
         # TODO: print out commit hashes for things we rebased but not accessible
@@ -390,11 +390,31 @@ def run(self) -> List[DiffMeta]:
                 # TODO: Do a separate check for this
                 if h.commit_id not in diff_meta_index:
                     continue
+                new_orig = diff_meta_index[h.commit_id].orig
                 self.check_invariants_for_diff(
                     h.commit_id,
-                    diff_meta_index[h.commit_id].orig,
+                    new_orig,
                     pre_branch_state_index.get(h.commit_id),
                 )
+                # Test that orig commits are accessible from HEAD, if the old
+                # commits were accessible.  And if the commit was not
+                # accessible, it better not be accessible now!
+                if self.sh.git(
+                    "merge-base", "--is-ancestor", h.commit_id, old_head, exitcode=True
+                ):
+                    assert new_head is not None
+                    assert self.sh.git(
+                        "merge-base", "--is-ancestor", new_orig, new_head, exitcode=True
+                    )
+                else:
+                    if new_head is not None:
+                        assert not self.sh.git(
+                            "merge-base",
+                            "--is-ancestor",
+                            new_orig,
+                            new_head,
+                            exitcode=True,
+                        )
 
         # NB: earliest first, which is the intuitive order for unit testing
         return list(reversed(diffs_to_submit))
@@ -1296,11 +1316,17 @@ def assert_eq(a: Any, b: Any) -> None:
         assert_eq(base_commit.tree, user_parent_commit.tree)
 
         # 6.  Orig commit was correctly pushed
-        assert_eq(orig_commit.commit_id, GitCommitHash(
-            self.sh.git(
-                "rev-parse", self.remote_name + "/" + branch_orig(self.username, elaborated_orig_diff.ghnum)
-            )
-        ))
+        assert_eq(
+            orig_commit.commit_id,
+            GitCommitHash(
+                self.sh.git(
+                    "rev-parse",
+                    self.remote_name
+                    + "/"
+                    + branch_orig(self.username, elaborated_orig_diff.ghnum),
+                )
+            ),
+        )
 
         # 7. Branches are either unchanged, or parent (no force pushes)
         # NB: head is always merged in as first parent
diff --git a/test_ghstack.py b/test_ghstack.py
index 09da546..5cee2d3 100644
--- a/test_ghstack.py
+++ b/test_ghstack.py
@@ -153,15 +153,7 @@ def gh(
         revs: Sequence[str] = (),
         stack: bool = True,
     ) -> List[ghstack.submit.DiffMeta]:
-        """
-        ghstack.submit.parse_revs(
-            revs,
-            base_ref="origin/master",
-            sh=self.sh,
-        )
-        """
-
-        kwargs = dict(
+        r = ghstack.submit.main(
             msg=msg,
             username="ezyang",
             github=self.github,
@@ -179,8 +171,8 @@ def gh(
             stack=stack,
             check_invariants=True,
         )
-
-        return ghstack.submit.main(**kwargs)
+        self.check_global_github_invariants()
+        return r
 
     def gh_land(self, pull_request: str) -> None:
         return ghstack.land.main(
@@ -201,6 +193,32 @@ def gh_unlink(self) -> None:
             remote_name="origin",
         )
 
+    def check_global_github_invariants(self) -> None:
+        r = self.github.graphql(
+            """
+          query {
+            repository(name: "pytorch", owner: "pytorch") {
+              pullRequests {
+                nodes {
+                  baseRefName
+                  headRefName
+                  closed
+                }
+              }
+            }
+          }
+        """
+        )
+        # No refs may be reused for multiple open PRs
+        seen_refs = set()
+        for pr in r["data"]["repository"]["pullRequests"]["nodes"]:
+            if pr["closed"]:
+                continue
+            assert pr["baseRefName"] not in seen_refs
+            seen_refs.add(pr["baseRefName"])
+            assert pr["headRefName"] not in seen_refs
+            seen_refs.add(pr["headRefName"])
+
     def dump_github(self) -> str:
         r = self.github.graphql(
             """