diff --git a/test/cli/backup-concurrency.test.ts b/test/cli/backup-concurrency.test.ts
index 964bff5..cd411d8 100644
--- a/test/cli/backup-concurrency.test.ts
+++ b/test/cli/backup-concurrency.test.ts
@@ -396,6 +396,69 @@ describe("lib.backup()", () => {
         }
     });
 
+    // A throw from `onProgress` fails the backup: `lib.backup()` rejects with
+    // it. It rejects only once it has finished with every original it was
+    // working on, so nothing is written into the backup after it has ended
+    // and removed its lock. Here, with a content pool of eight, all eight
+    // downloads wait at once. Photo 103's is let go first and answered 404,
+    // and the line reporting that failure throws while the other seven wait.
+    it("when onProgress throws, rejects only once the originals it was working on are in place", async () => {
+        const dir = mkdtempSync(join(root, "backup-"));
+        const holding = holdingServer(103);
+        const lib = await openLibrary(
+            dir,
+            holding.fetch,
+            new RequestPools({ contentConcurrency: 8 }),
+        );
+        const lines: string[] = [];
+        try {
+            const backup = lib.backup({
+                onProgress: (line) => {
+                    lines.push(line);
+                    if (line.startsWith("FAILED")) {
+                        throw new Error("the progress output is closed");
+                    }
+                },
+            });
+            // The photos whose originals are in place, and whether the lock
+            // is there, at the moment `lib.backup()` rejects.
+            let storedWhenRejected: number[] = [];
+            let lockedWhenRejected = true;
+            const rejection = backup.catch((err: unknown) => {
+                storedWhenRejected = PHOTO_IDS.filter((id) =>
+                    existsSync(savedAt(dir, id)),
+                );
+                lockedWhenRejected = existsSync(join(dir, "backup.lock"));
+                return err;
+            });
+
+            await vi.waitFor(() => expect(holding.held).toHaveLength(8), {
+                interval: 5,
+                timeout: 5000,
+            });
+            const at103 = holding.held.findIndex((h) => h.id === 103);
+            holding.held.splice(at103, 1)[0]!.release();
+            await vi.waitFor(
+                () =>
+                    expect(
+                        lines.some((line) => line.startsWith("FAILED")),
+                    ).toBe(true),
+                { interval: 5, timeout: 5000 },
+            );
+            for (const download of holding.held) download.release();
+
+            expect(await rejection).toEqual(
+                new Error("the progress output is closed"),
+            );
+            expect(storedWhenRejected).toEqual(
+                PHOTO_IDS.filter((id) => id !== 103),
+            );
+            expect(lockedWhenRejected).toBe(false);
+        } finally {
+            await lib.close();
+        }
+    });
+
     // Working on several originals at once changes only how long a backup
     // takes. The same two runs, with a content pool of one, which backs up
     // one original at a time, and with the default pools, end the same way: