diff --git a/extension/background/service-worker.js b/extension/background/service-worker.js
index 060e74af1..9843e6f1a 100644
--- a/extension/background/service-worker.js
+++ b/extension/background/service-worker.js
@@ -58,7 +58,7 @@ async function buildScanConfig() {
 // engages with the extension (DevTools panel/sidebar opened, popup scan, etc).
 async function ensureContentScriptInjected(tabId) {
   const state = getState(tabId);
-  if (state.csInjected) return true;
+  if (state.csInjected) return { ok: true };
   try {
     await chrome.scripting.executeScript({
       target: { tabId },
@@ -66,16 +66,29 @@ async function ensureContentScriptInjected(tabId) {
       injectImmediately: true,
     });
     state.csInjected = true;
-    return true;
+    return { ok: true };
   } catch (err) {
-    // Common cause: chrome:// pages, the web store, or other restricted URLs
-    return false;
+    // Common cause: chrome:// pages, the web store, the Chrome Web Store, or
+    // file:// pages when "Allow access to file URLs" is off. Keep the real
+    // error so the UI can explain what happened.
+    return { ok: false, error: err?.message || String(err) };
   }
 }
 
 async function sendScanToTab(tabId) {
-  const ok = await ensureContentScriptInjected(tabId);
-  if (!ok) return;
+  const { ok, error } = await ensureContentScriptInjected(tabId);
+  if (!ok) {
+    // Injection was blocked. Tell an open popup why so it can stop showing
+    // "Scanning..." and surface a hint. The popup may be closed, so ignore
+    // delivery failures.
+    let url = '';
+    try { url = (await chrome.tabs.get(tabId))?.url || ''; } catch { /* tab gone */ }
+    const message = url.startsWith('file:')
+      ? 'Can\u2019t scan local files. Enable \u201CAllow access to file URLs\u201D for Impeccable in chrome://extensions.'
+      : `Couldn\u2019t scan this page${error ? `: ${error}` : '.'}`;
+    chrome.runtime.sendMessage({ action: 'scan-failed', tabId, message }).catch(() => {});
+    return;
+  }
   const config = await buildScanConfig();
   chrome.tabs.sendMessage(tabId, { action: 'scan', config }).catch(() => {});
 }
diff --git a/extension/popup/popup.css b/extension/popup/popup.css
index 1cc43714d..85e13aac9 100644
--- a/extension/popup/popup.css
+++ b/extension/popup/popup.css
@@ -89,6 +89,17 @@ h1 {
   margin-bottom: 16px;
 }
 
+.scan-error {
+  margin: -8px 0 16px;
+  font-size: 11px;
+  line-height: 1.4;
+  color: oklch(70% 0.13 30);
+}
+
+.scan-error[hidden] {
+  display: none;
+}
+
 .btn {
   display: block;
   width: 100%;
diff --git a/extension/popup/popup.html b/extension/popup/popup.html
index 82c4b6388..e382231bb 100644
--- a/extension/popup/popup.html
+++ b/extension/popup/popup.html
@@ -25,6 +25,8 @@
     <button class="btn btn-secondary" id="btn-toggle">Hide overlays</button>
   </div>
 
+  <p class="scan-error" id="scan-error" hidden></p>
+
   <footer>
     <a href="https://impeccable.style" id="link-site">impeccable.style</a>
   </footer>
diff --git a/extension/popup/popup.js b/extension/popup/popup.js
index c8779df13..54a5a3822 100644
--- a/extension/popup/popup.js
+++ b/extension/popup/popup.js
@@ -8,8 +8,13 @@ const countNumber = document.getElementById('count-number');
 const countLabel = document.getElementById('count-label');
 const btnScan = document.getElementById('btn-scan');
 const btnToggle = document.getElementById('btn-toggle');
+const scanError = document.getElementById('scan-error');
 
 let overlaysVisible = true;
+// The popup only ever reflects the active tab. Broadcasts from the service
+// worker carry a tabId, so we cache the active one and ignore updates meant
+// for other tabs (e.g. a DevTools-driven rescan failing on a background tab).
+let activeTabId = null;
 
 async function getActiveTabId() {
   const [tab] = await chrome.tabs.query({ active: true, currentWindow: true });
@@ -29,11 +34,14 @@ function updateFromState(state) {
 async function loadState() {
   const tabId = await getActiveTabId();
   if (!tabId) return;
+  activeTabId = tabId;
   chrome.runtime.sendMessage({ action: 'get-state', tabId }, updateFromState);
 }
 
 // Listen for real-time updates from service worker
 chrome.runtime.onMessage.addListener((msg) => {
+  // Ignore broadcasts for a tab other than the one this popup is showing.
+  if (msg.tabId != null && activeTabId != null && msg.tabId !== activeTabId) return;
   if (msg.action === 'findings-updated') {
     const count = msg.findings?.reduce((sum, f) => sum + f.findings.length, 0) || 0;
     countNumber.textContent = String(count);
@@ -41,6 +49,13 @@ chrome.runtime.onMessage.addListener((msg) => {
     countLabel.textContent = count === 1 ? 'anti-pattern' : 'anti-patterns';
     btnScan.textContent = 'Scan page';
     btnScan.disabled = false;
+    scanError.hidden = true;
+  }
+  if (msg.action === 'scan-failed') {
+    btnScan.textContent = 'Scan page';
+    btnScan.disabled = false;
+    scanError.textContent = msg.message || 'Couldn\u2019t scan this page.';
+    scanError.hidden = false;
   }
   if (msg.action === 'overlays-toggled-broadcast') {
     overlaysVisible = msg.visible;
@@ -51,6 +66,8 @@ chrome.runtime.onMessage.addListener((msg) => {
 btnScan.addEventListener('click', async () => {
   const tabId = await getActiveTabId();
   if (!tabId) return;
+  activeTabId = tabId;
+  scanError.hidden = true;
   btnScan.textContent = 'Scanning...';
   btnScan.disabled = true;
   chrome.runtime.sendMessage({ action: 'scan', tabId });