diff --git a/demos/_unit-test/late-output-error.php b/demos/_unit-test/late-output-error.php
index 4b9b371008..f9628205e6 100644
--- a/demos/_unit-test/late-output-error.php
+++ b/demos/_unit-test/late-output-error.php
@@ -4,25 +4,27 @@
 
 namespace Atk4\Ui\Demos;
 
+use Atk4\Ui\Button;
+use Atk4\Ui\Callback;
 use Atk4\Ui\CallbackLater;
+use Atk4\Ui\Header;
+use Atk4\Ui\Modal;
 
 /** @var \Atk4\Ui\App $app */
 require_once __DIR__ . '/../init-app.php';
 
-$cb = CallbackLater::addTo($app);
-$cb->setUrlTrigger('err_headers_already_sent');
-
-$modal = \Atk4\Ui\Modal::addTo($app, ['cb' => $cb]);
-$modal->set(function () {
+$cbH1 = Callback::addTo($app);
+$cbH1->setUrlTrigger('err_headers_already_sent_1');
+$modalH1 = Modal::addTo($app, ['cb' => $cbH1]);
+$modalH1->set(function () {
     header('x-unmanaged-header: test');
     flush();
 });
 
-$cb2 = CallbackLater::addTo($app);
-$cb2->setUrlTrigger('err_unexpected_output_detected');
-
-$modal2 = \Atk4\Ui\Modal::addTo($app, ['cb' => $cb2]);
-$modal2->set(function () {
+$cbO1 = Callback::addTo($app);
+$cbO1->setUrlTrigger('err_unexpected_output_detected_1');
+$modalO1 = Modal::addTo($app, ['cb' => $cbO1]);
+$modalO1->set(function () {
     // unexpected output can be detected only when output buffering is enabled and not flushed
     if (ob_get_level() === 0) {
         ob_start();
@@ -30,8 +32,28 @@
     echo 'unmanaged output';
 });
 
-$button = \Atk4\Ui\Button::addTo($app, ['Test LateOutputError: Headers already sent']);
-$button->on('click', $modal->show());
+$cbH2 = CallbackLater::addTo($app);
+$cbH2->setUrlTrigger('err_headers_already_sent_2');
+$modalH2 = Modal::addTo($app, ['cb' => $cbH2]);
+$modalH2->set($modalH1->fx);
+
+$cbO2 = CallbackLater::addTo($app);
+$cbO2->setUrlTrigger('err_unexpected_output_detected_2');
+$modalO2 = Modal::addTo($app, ['cb' => $cbO2]);
+$modalO2->set($modalO1->fx);
+
+Header::addTo($app, ['content' => 'Before render (/w Callback)']);
+
+$buttonH1 = Button::addTo($app, ['Test LateOutputError I: Headers already sent']);
+$buttonH1->on('click', $modalH1->show());
+
+$buttonO1 = Button::addTo($app, ['Test LateOutputError I: Unexpected output detected']);
+$buttonO1->on('click', $modalO1->show());
+
+Header::addTo($app, ['content' => 'After render (/w CallbackLater)']);
+
+$buttonH2 = Button::addTo($app, ['Test LateOutputError II: Headers already sent']);
+$buttonH2->on('click', $modalH2->show());
 
-$button2 = \Atk4\Ui\Button::addTo($app, ['Test LateOutputError: Unexpected output detected']);
-$button2->on('click', $modal2->show());
+$buttonO2 = Button::addTo($app, ['Test LateOutputError II: Unexpected output detected']);
+$buttonO2->on('click', $modalO2->show());
diff --git a/src/App.php b/src/App.php
index ccf8ff6939..82ac2709d3 100644
--- a/src/App.php
+++ b/src/App.php
@@ -407,7 +407,7 @@ public function terminate($output = '', array $headers = []): void
             $this->outputResponse($output, $headers);
         }
 
-        $this->runCalled = true; // prevent shutdown function from triggering.
+        $this->runCalled = true; // prevent shutdown function from triggering
         $this->callExit();
     }
 
@@ -1116,6 +1116,8 @@ protected function outputLateOutputError(LateOutputError $exception): void
 
         $this->outputResponseUnsafe($plainTextMessage, $headersNew);
 
+        $this->runCalled = true; // prevent shutdown function from triggering
+
         exit(1); // should be never reached from phpunit because we set catchExceptions = false
     }
 
diff --git a/tests/DemosHttpTest.php b/tests/DemosHttpTest.php
index 4d95b1118c..1b4810ea35 100644
--- a/tests/DemosHttpTest.php
+++ b/tests/DemosHttpTest.php
@@ -114,32 +114,30 @@ protected function getPathWithAppVars(string $path): string
         return parent::getPathWithAppVars($path);
     }
 
-    private function getLateOutputErrorPath(string $trigger): string
+    /**
+     * @dataProvider demoLateOutputErrorProvider
+     */
+    public function testDemoLateOutputError(string $urlTrigger, string $expectedOutput): void
     {
-        return '_unit-test/late-output-error.php?' . Callback::URL_QUERY_TRIGGER_PREFIX . $trigger . '=ajax&'
-            . Callback::URL_QUERY_TARGET . '=' . $trigger . '&__atk_json=1';
-    }
+        $path = '_unit-test/late-output-error.php?' . Callback::URL_QUERY_TRIGGER_PREFIX . $urlTrigger . '=ajax&'
+            . Callback::URL_QUERY_TARGET . '=' . $urlTrigger . '&__atk_json=1';
 
-    public function testDemoLateOutputErrorHeadersAlreadySent(): void
-    {
-        $response = $this->getResponseFromRequest5xx($this->getLateOutputErrorPath('err_headers_already_sent'));
+        $response = $this->getResponseFromRequest5xx($path);
 
         $this->assertSame(500, $response->getStatusCode());
-        $this->assertSame(
-            "\n" . '!! FATAL UI ERROR: Headers already sent, more headers cannot be set at this stage !!' . "\n",
-            $response->getBody()->getContents()
-        );
+        $this->assertSame($expectedOutput, $response->getBody()->getContents());
     }
 
-    public function testDemoLateOutputErrorUnexpectedOutputDetected(): void
+    public function demoLateOutputErrorProvider(): array
     {
-        $response = $this->getResponseFromRequest5xx($this->getLateOutputErrorPath('err_unexpected_output_detected'));
-
-        $this->assertSame(500, $response->getStatusCode());
-        $this->assertSame(
-            'unmanaged output'
-                . "\n" . '!! FATAL UI ERROR: Unexpected output detected !!' . "\n",
-            $response->getBody()->getContents()
-        );
+        $hOutput = "\n" . '!! FATAL UI ERROR: Headers already sent, more headers cannot be set at this stage !!' . "\n";
+        $oOutput = 'unmanaged output' . "\n" . '!! FATAL UI ERROR: Unexpected output detected !!' . "\n";
+
+        return [
+            ['err_headers_already_sent_2', $hOutput],
+            ['err_unexpected_output_detected_2', $oOutput],
+            ['err_headers_already_sent_1', $hOutput],
+            ['err_unexpected_output_detected_1', $oOutput],
+        ];
     }
 }