diff --git a/e2e/app.test.ts b/e2e/app.test.ts
index 7fc2ab9..a36347d 100644
--- a/e2e/app.test.ts
+++ b/e2e/app.test.ts
@@ -188,9 +188,9 @@ describeEngines((engine) => {
     await page.click("#cancel");
 
     await page.waitForSelector('[data-message="error"]');
-    expect(await page.textContent('[data-message="error"]')).toContain(
-      "cancelled",
-    );
+    const message = await page.textContent('[data-message="error"]');
+    expect(message).toContain("cancelled");
+    expect(message).toContain("incomplete");
     await page.waitForSelector('[role="progressbar"]', { state: "detached" });
     await page.waitForSelector("#export:not([disabled])");
   });
diff --git a/src/main.ts b/src/main.ts
index 70b2594..7143fc0 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -158,7 +158,14 @@ function start(app: HTMLElement) {
         form.message = null;
       }, SAVED_MESSAGE_MS);
     } catch (err) {
-      form.message = { kind: "error", text: describe(err) };
+      // The download had already started, and iOS keeps whatever was flushed.
+      const reached = Math.round(form.progress * 100);
+      form.message = {
+        kind: "error",
+        text:
+          `${describe(err)} (stopped at ${reached}%). ` +
+          "Any file that was downloaded is incomplete — delete it and try again.",
+      };
     } finally {
       form.exporting = false;
       form.progress = 0;