diff --git a/src/aphront/response/Aphront403Response.php b/src/aphront/response/Aphront403Response.php
--- a/src/aphront/response/Aphront403Response.php
+++ b/src/aphront/response/Aphront403Response.php
@@ -25,10 +25,17 @@
     $request = $this->getRequest();
     $user = $request->getUser();
 
+    $is_serious = PhabricatorEnv::getEnvConfig('phabricator.serious-business');
+    if ($is_serious) {
+      $button_text = pht('Back');
+    } else {
+      $button_text = pht('Peace Out');
+    }
+
     $dialog = id(new AphrontDialogView())
       ->setUser($user)
       ->setTitle(pht('403 Forbidden'))
-      ->addCancelButton('/', pht('Peace Out'))
+      ->addCancelButton('/', $button_text)
       ->appendParagraph($forbidden_text);
 
     $view = id(new PhabricatorStandardPageView())
diff --git a/src/aphront/response/Aphront404Response.php b/src/aphront/response/Aphront404Response.php
--- a/src/aphront/response/Aphront404Response.php
+++ b/src/aphront/response/Aphront404Response.php
@@ -10,14 +10,24 @@
     $request = $this->getRequest();
     $user = $request->getUser();
 
+    $is_serious = PhabricatorEnv::getEnvConfig('phabricator.serious-business');
+    if ($is_serious) {
+      $body = pht(
+        'The link you clicked may be misspelled or the page may '.
+        'have been removed.');
+      $button_text = pht('Back');
+    } else {
+      $body = pht(
+        'Do not dwell in the past, do not dream of the future, '.
+        'concentrate the mind on the present moment.');
+      $button_text = pht('Focus');
+    }
+
     $dialog = id(new AphrontDialogView())
       ->setUser($user)
       ->setTitle(pht('404 Not Found'))
-      ->addCancelButton('/', pht('Focus'))
-      ->appendParagraph(
-        pht(
-          'Do not dwell in the past, do not dream of the future, '.
-          'concentrate the mind on the present moment.'));
+      ->addCancelButton('/', $button_text)
+      ->appendParagraph($body);
 
     $view = id(new PhabricatorStandardPageView())
       ->setTitle(pht('404 Not Found'))