diff --git a/install.ps1 b/install.ps1
index 63815cec0..f0549a723 100644
--- a/install.ps1
+++ b/install.ps1
@@ -16,7 +16,7 @@
     # and Invoke-WebRequest throws on a cut transfer before the hash is read:
     # the site's build rewrites this line there (scripts/sitebootstrap.py).
     $mismatchAdvice = "The download was most likely cut short: try again. Within minutes of an update of this project the two files can come from caches of different ages. Waiting five minutes settles that."
-    $defaultInstallSha256 = "65f7d49de83de9d99734293009762adaff6f844c2c3627f507d4dc3b029f57be"
+    $defaultInstallSha256 = "fa8d130b046b1c1d1bc2e8da49905a7572d81ee95d023b483cdcf009a1d4a64c"
     $maximumInstallerBytes = 2MB
     $installerExit = 0
     $installer = if ($PSScriptRoot) { Join-Path $PSScriptRoot "install.py" } else { $null }
diff --git a/install.py b/install.py
index 12cea69cf..f887c308b 100644
--- a/install.py
+++ b/install.py
@@ -5759,8 +5759,9 @@ def _download_one(
             reason = f"HTTP {exc.code}"
             if exc.code in (403, 429):
                 reason += " (GitHub rate limit or access refused)"
-            if exc.code >= 500 or exc.code == 429:
-                reason = NetworkReason(reason)
+            reason = (NetworkReason if exc.code >= 500 or exc.code == 429
+                      else HttpReason)(reason)
+            reason.url = url
             if verbose:
                 print(f"    Attempt {attempt} failed: {exc}", file=sys.stderr)
             if tmp_path is not None and partial is None and tmp_path.exists():
@@ -6264,6 +6265,32 @@ class NetworkReason(str):
     """A failure the connection caused: cut, timed out, refused by a server
     or a proxy. The summary names the network instead of the folder."""
 
+    url = ""
+
+
+class HttpReason(str):
+    """A file the server answered for with an error of its own (HTTP 404):
+    the summary names the address, where "HTTP 404" alone told the user
+    neither what failed nor that running again changes nothing."""
+
+    url = ""
+
+
+def _http_failure_lines(failures: list[tuple[str, str]]) -> list[str]:
+    """The address the first refused file was asked at, and what chose it:
+    a mirror named with RETROBIOS_BASE_URL that holds the lists and not yet
+    the files, or a list out of step with the files it names."""
+    refused = [reason for _dest, reason in failures if isinstance(reason, HttpReason)]
+    if not refused:
+        return []
+    more = f" (and {_plural(len(refused) - 1, 'other file')})" if len(refused) > 1 else ""
+    lines = [f"  The server answered {refused[0]} for {refused[0].url}{more}."]
+    if os.environ.get("RETROBIOS_BASE_URL"):
+        return lines + _address_lines(refused[0].url)[1:]
+    return lines + ["  The file list names files the server does not hold, and running "
+                    "again changes nothing until the two match.",
+                    f"  More help: {TROUBLESHOOTING_PAGE}"]
+
 
 def _transfer_failure(exc: BaseException) -> bool:
     """Whether a download error came from the connection, not the disk."""
@@ -6409,8 +6436,12 @@ def _report_outcome(
     if any(reason == READ_ONLY for _dest, reason in failures or []):
         advice.append("  Files marked read-only are in the way: clear that attribute "
                       "(or delete them), then run again.")
+    advice += _http_failure_lines(failures or [])
     if any(isinstance(reason, NetworkReason) for _dest, reason in failures or []):
         advice.append(NETWORK_FAILURE_ADVICE)
+        if os.environ.get("RETROBIOS_BASE_URL"):
+            # A mirror that is down read as this machine's connection.
+            advice += _address_lines(BASE_URL)[1:]
     if any(str(reason).startswith(NOT_KEPT) for _dest, reason in failures or []):
         # The same run fails the same way: "retry only those" sent a phone
         # user round the loop with nothing changed.
@@ -6472,7 +6503,7 @@ def _report_outcome(
 
     if errors and not downloaded:
         say(f"\nNothing was installed. {_plural(errors, 'file')} failed.")
-        if failures and all(isinstance(r, (NetworkReason, OfflineReason))
+        if failures and all(isinstance(r, (NetworkReason, HttpReason, OfflineReason))
                             for _d, r in failures):
             # The folder was never the problem: naming it as the place being
             # written to sent people looking at their disk.
diff --git a/install.sh b/install.sh
index 6f2d73596..17fab0cf5 100755
--- a/install.sh
+++ b/install.sh
@@ -23,7 +23,7 @@ DEFAULT_INSTALL_URL="https://raw.githubusercontent.com/Abdess/retrobios/main/ins
 # curl or wget stop on a cut transfer before the hash is read: the site's
 # build rewrites this line there (scripts/sitebootstrap.py).
 MISMATCH_ADVICE="The download was most likely cut short: try again. Within minutes of an update of this project the two files can come from caches of different ages. Waiting five minutes settles that."
-DEFAULT_INSTALL_SHA256="65f7d49de83de9d99734293009762adaff6f844c2c3627f507d4dc3b029f57be"
+DEFAULT_INSTALL_SHA256="fa8d130b046b1c1d1bc2e8da49905a7572d81ee95d023b483cdcf009a1d4a64c"
 MAX_INSTALLER_BYTES=2097152
 
 cleanup() {
diff --git a/install/launchers/index.json b/install/launchers/index.json
index 578c43e15..40d7f1510 100644
--- a/install/launchers/index.json
+++ b/install/launchers/index.json
@@ -27,8 +27,8 @@
         {
           "dest": "roms/ports/retrobios/install.py",
           "repo_path": "install.py",
-          "size": 1094917,
-          "sha256": "65f7d49de83de9d99734293009762adaff6f844c2c3627f507d4dc3b029f57be",
+          "size": 1096372,
+          "sha256": "fa8d130b046b1c1d1bc2e8da49905a7572d81ee95d023b483cdcf009a1d4a64c",
           "executable": false
         },
         {
@@ -71,8 +71,8 @@
     "muos": {
       "archive": {
         "dest": "ARCHIVE/RetroBIOS.muxapp",
-        "size": 1148544,
-        "sha256": "ba22f19af63c4003f7195636f6c7a5ebe3900ce6a300fe089058ff78eff52eb2"
+        "size": 1149999,
+        "sha256": "143e0067766ccb85f299ca125c1dbdaa5ebec15e85ea49a9e354a677fb7f931d"
       },
       "members": [
         {
@@ -92,8 +92,8 @@
         {
           "name": "RetroBIOS/install.py",
           "repo_path": "install.py",
-          "size": 1094917,
-          "sha256": "65f7d49de83de9d99734293009762adaff6f844c2c3627f507d4dc3b029f57be",
+          "size": 1096372,
+          "sha256": "fa8d130b046b1c1d1bc2e8da49905a7572d81ee95d023b483cdcf009a1d4a64c",
           "executable": false
         },
         {
@@ -166,8 +166,8 @@
         {
           "dest": ".config/modules/retrobios/install.py",
           "repo_path": "install.py",
-          "size": 1094917,
-          "sha256": "65f7d49de83de9d99734293009762adaff6f844c2c3627f507d4dc3b029f57be",
+          "size": 1096372,
+          "sha256": "fa8d130b046b1c1d1bc2e8da49905a7572d81ee95d023b483cdcf009a1d4a64c",
           "executable": false
         },
         {
diff --git a/tests/test_install.py b/tests/test_install.py
index 9b4d80a6a..4f75d11cf 100644
--- a/tests/test_install.py
+++ b/tests/test_install.py
@@ -5160,6 +5160,54 @@ class ANetworkFailureNamesTheNetwork(unittest.TestCase):
         self.assertNotIn("being written to", text)
 
 
+class AnHttpErrorNamesItsAddress(unittest.TestCase):
+    """A file the server answered 404 for ended on "HTTP 404" alone: no
+    address, no cause, and a mirror chosen with RETROBIOS_BASE_URL went
+    unnamed, as it did behind a NETWORK failure of that mirror."""
+
+    def _said(self, failures: list, base_url: str = "") -> str:
+        out = io.StringIO()
+        env = {"RETROBIOS_BASE_URL": base_url} if base_url else {}
+        with unittest.mock.patch.dict(os.environ, env), contextlib.redirect_stdout(out), \
+                self.assertRaises(SystemExit) as raised:
+            install._report_outcome([Path("/bios")], 0, 0, len(failures), 0, False,
+                                    failures=failures)
+        self.assertEqual(raised.exception.code, 2)
+        return " ".join(out.getvalue().split())
+
+    def _reason(self, kind: type, words: str, url: str) -> str:
+        reason = kind(words)
+        reason.url = url
+        return reason
+
+    def test_the_address_and_the_cause_are_said(self):
+        url = "https://raw.githubusercontent.com/Abdess/retrobios/main/bios/a.bin"
+        failures = [("a.bin", self._reason(install.HttpReason, "HTTP 404", url)),
+                    ("b.bin", self._reason(install.HttpReason, "HTTP 404", url + "x"))]
+        with unittest.mock.patch.dict(os.environ):
+            os.environ.pop("RETROBIOS_BASE_URL", None)
+            text = self._said(failures)
+        self.assertIn(f"The server answered HTTP 404 for {url} (and 1 other file).", text)
+        self.assertIn("running again changes nothing", text)
+        self.assertIn("BIOS folder: /bios", text)
+        self.assertNotIn("RETROBIOS_BASE_URL", text)
+
+    def test_a_mirror_named_in_the_environment_is_said(self):
+        mirror = "http://127.0.0.1:18766"
+        for kind, words in ((install.HttpReason, "HTTP 404"),
+                            (install.NetworkReason, "the connection timed out")):
+            with self.subTest(kind=kind.__name__):
+                text = self._said([("a.bin", self._reason(kind, words, f"{mirror}/a.bin"))],
+                                  base_url=mirror)
+                self.assertIn("RETROBIOS_BASE_URL is set in this environment", text)
+
+    def test_the_download_records_the_address(self):
+        error = urllib.error.HTTPError("https://x/bios/a.bin", 404, "Not Found", None, None)
+        reason = ANetworkFailureNamesTheNetwork._fail(self, error)
+        self.assertIsInstance(reason, install.HttpReason)
+        self.assertTrue(reason.url.endswith("/bios/a.bin"), reason.url)
+
+
 class RetroBatIsFoundWhereItSaysItIs(unittest.TestCase):
     """RetroBat on D:\\RetroBat was not detected, and --platform retrobat
     wrote 7.6 GB into C:\\RetroBat\\bios, a folder it does not read.