diff --git a/install.ps1 b/install.ps1
index 013467a5..2ab5bf18 100644
--- a/install.ps1
+++ b/install.ps1
@@ -11,7 +11,7 @@
 
     $ErrorActionPreference = "Stop"
     $defaultInstallUrl = "https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-    $defaultInstallSha256 = "568d9399c5639db607cfc3944fa46a0dfa8d47741291d9346ed336cd2500eb00"
+    $defaultInstallSha256 = "851c1d9e8b2b481adf50d1af2c67d1e44d9b50504fdef3d5b6514332ac4457eb"
     $maximumInstallerBytes = 2MB
     $installerExit = 0
     $installer = if ($PSScriptRoot) { Join-Path $PSScriptRoot "install.py" } else { $null }
diff --git a/install.py b/install.py
index bc0b9076..74807803 100644
--- a/install.py
+++ b/install.py
@@ -6394,6 +6394,12 @@ def _found_elsewhere(
 ) -> tuple[str, str]:
     """(reason, folder too deep) for an entry missing at its place."""
     dest = str(entry.get("dest", ""))
+    if entry.get("case_variant"):
+        # A pack is read on Windows and macOS too, where two names that
+        # differ only by case are one file: it carries one of the pair. Moving
+        # the other copy here would take it from the core that opens it.
+        return ("a second name the pack leaves out: Windows and macOS see it "
+                "as the same file. Run the command without --check to add it"), ""
     name = dest.rsplit("/", 1)[-1]
     sizes = {copy.get("size") for copy in _copies(entry) if isinstance(copy.get("size"), int)}
     sha1s = {copy.get("sha1") for copy in _copies(entry) if copy.get("sha1")}
@@ -7294,7 +7300,7 @@ def _install(args: argparse.Namespace, real_stdout: TextIO) -> None:
         if variants and case_kept:
             # The list is folded for Windows and macOS, but here bios.col and
             # BIOS.col are two files that two cores open.
-            files = files + variants
+            files = files + [{**variant, "case_variant": True} for variant in variants]
         if not case_kept:
             files, folded = fold_case_twins(files)
             if folded:
diff --git a/install.sh b/install.sh
index 999f63c0..b3217604 100755
--- a/install.sh
+++ b/install.sh
@@ -18,7 +18,7 @@ esac
 TEMP_INSTALLER=""
 TEMP_DIRECTORY=""
 DEFAULT_INSTALL_URL="https://raw.githubusercontent.com/Abdess/retrobios/main/install.py"
-DEFAULT_INSTALL_SHA256="568d9399c5639db607cfc3944fa46a0dfa8d47741291d9346ed336cd2500eb00"
+DEFAULT_INSTALL_SHA256="851c1d9e8b2b481adf50d1af2c67d1e44d9b50504fdef3d5b6514332ac4457eb"
 MAX_INSTALLER_BYTES=2097152
 
 cleanup() {
diff --git a/tests/test_install_offline.py b/tests/test_install_offline.py
index ad555075..834e320a 100644
--- a/tests/test_install_offline.py
+++ b/tests/test_install_offline.py
@@ -168,6 +168,23 @@ class CheckNamesWhyAFileIsRefused(Scratch):
         self.place("GBA_BIOS.BIN", GBA)
         self.assertIn("case of the name: the folder holds GBA_BIOS.BIN", self.check())
 
+    def test_a_case_twin_the_pack_leaves_out_is_not_a_wrong_folder(self):
+        """np2kai/2608_bd.wav and np2/2608_bd.wav hold the same bytes. Told
+        "wrong folder", a user moved np2's copy and broke that core."""
+        probe = self.root / "CaseProbe"
+        probe.write_text("x")
+        if (self.root / "caseprobe").exists():
+            self.skipTest("this filesystem does not keep case")
+        data = json.loads(self.list.read_text())
+        twin = next(f for f in data["files"] if f["dest"] == "gba_bios.bin")
+        data["case_variants"] = [{**twin, "dest": "twin/gba_bios.bin"}]
+        self.list.write_text(json.dumps(data))
+        self.complete()
+        out = self.check()
+        self.assertIn("missing: twin/gba_bios.bin", out)
+        self.assertIn("a second name the pack leaves out", out)
+        self.assertNotIn("wrong folder: it is at gba_bios.bin", out)
+
     def test_a_nested_bios_folder_is_named(self):
         for name, data in PAYLOAD.items():
             self.place(f"system/{name}", data)