diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -2055,6 +2055,8 @@
     'PhabricatorApplicationTransactionValidationResponse' => 'applications/transactions/response/PhabricatorApplicationTransactionValidationResponse.php',
     'PhabricatorApplicationTransactionValueController' => 'applications/transactions/controller/PhabricatorApplicationTransactionValueController.php',
     'PhabricatorApplicationTransactionView' => 'applications/transactions/view/PhabricatorApplicationTransactionView.php',
+    'PhabricatorApplicationTransactionWarningException' => 'applications/transactions/exception/PhabricatorApplicationTransactionWarningException.php',
+    'PhabricatorApplicationTransactionWarningResponse' => 'applications/transactions/response/PhabricatorApplicationTransactionWarningResponse.php',
     'PhabricatorApplicationUninstallController' => 'applications/meta/controller/PhabricatorApplicationUninstallController.php',
     'PhabricatorApplicationsApplication' => 'applications/meta/application/PhabricatorApplicationsApplication.php',
     'PhabricatorApplicationsController' => 'applications/meta/controller/PhabricatorApplicationsController.php',
@@ -7484,6 +7486,8 @@
     'PhabricatorApplicationTransactionValidationResponse' => 'AphrontProxyResponse',
     'PhabricatorApplicationTransactionValueController' => 'PhabricatorApplicationTransactionController',
     'PhabricatorApplicationTransactionView' => 'AphrontView',
+    'PhabricatorApplicationTransactionWarningException' => 'Exception',
+    'PhabricatorApplicationTransactionWarningResponse' => 'AphrontProxyResponse',
     'PhabricatorApplicationUninstallController' => 'PhabricatorApplicationsController',
     'PhabricatorApplicationsApplication' => 'PhabricatorApplication',
     'PhabricatorApplicationsController' => 'PhabricatorController',
diff --git a/src/applications/transactions/editengine/PhabricatorEditEngine.php b/src/applications/transactions/editengine/PhabricatorEditEngine.php
--- a/src/applications/transactions/editengine/PhabricatorEditEngine.php
+++ b/src/applications/transactions/editengine/PhabricatorEditEngine.php
@@ -1968,6 +1968,7 @@
       ->setContinueOnNoEffect($request->isContinueRequest())
       ->setContinueOnMissingFields(true)
       ->setContentSourceFromRequest($request)
+      ->setRaiseWarnings(!$request->getBool('editEngine.warnings'))
       ->setIsPreview($is_preview);
 
     try {
@@ -1980,6 +1981,10 @@
       return id(new PhabricatorApplicationTransactionNoEffectResponse())
         ->setCancelURI($view_uri)
         ->setException($ex);
+    } catch (PhabricatorApplicationTransactionWarningException $ex) {
+      return id(new PhabricatorApplicationTransactionWarningResponse())
+        ->setCancelURI($view_uri)
+        ->setException($ex);
     }
 
     if (!$is_preview) {
diff --git a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
--- a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
+++ b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
@@ -48,6 +48,7 @@
   private $mentionedPHIDs;
   private $continueOnNoEffect;
   private $continueOnMissingFields;
+  private $raiseWarnings;
   private $parentMessageID;
   private $heraldAdapter;
   private $heraldTranscript;
@@ -273,6 +274,15 @@
     return $this->applicationEmail;
   }
 
+  public function setRaiseWarnings($raise_warnings) {
+    $this->raiseWarnings = $raise_warnings;
+    return $this;
+  }
+
+  public function getRaiseWarnings() {
+    return $this->raiseWarnings;
+  }
+
   public function getTransactionTypesForObject($object) {
     $old = $this->object;
     try {
@@ -919,6 +929,19 @@
         throw new PhabricatorApplicationTransactionValidationException($errors);
       }
 
+      if ($this->raiseWarnings) {
+        $warnings = array();
+        foreach ($xactions as $xaction) {
+          if ($this->hasWarnings($object, $xaction)) {
+            $warnings[] = $xaction;
+          }
+        }
+        if ($warnings) {
+          throw new PhabricatorApplicationTransactionWarningException(
+            $warnings);
+        }
+      }
+
       $this->willApplyTransactions($object, $xactions);
 
       if ($object->getID()) {
@@ -4277,4 +4300,28 @@
     }
   }
 
+  private function hasWarnings($object, $xaction) {
+    // TODO: For the moment, this is a very un-modular hack to support
+    // exactly one type of warning (mentioning users on a draft revision)
+    // that we want to show. See PHI433.
+
+    if (!($object instanceof DifferentialRevision)) {
+      return false;
+    }
+
+    if (!$object->isDraft()) {
+      return false;
+    }
+
+    $type = $xaction->getTransactionType();
+    if ($type != PhabricatorTransactions::TYPE_SUBSCRIBERS) {
+      return false;
+    }
+
+    // NOTE: This will currently warn even if you're only removing
+    // subscribers.
+
+    return true;
+  }
+
 }
diff --git a/src/applications/transactions/exception/PhabricatorApplicationTransactionWarningException.php b/src/applications/transactions/exception/PhabricatorApplicationTransactionWarningException.php
new file mode 100644
--- /dev/null
+++ b/src/applications/transactions/exception/PhabricatorApplicationTransactionWarningException.php
@@ -0,0 +1,13 @@
+<?php
+
+final class PhabricatorApplicationTransactionWarningException
+  extends Exception {
+
+  private $xactions;
+
+  public function __construct(array $xactions) {
+    $this->xactions = $xactions;
+    parent::__construct();
+  }
+
+}
diff --git a/src/applications/transactions/response/PhabricatorApplicationTransactionWarningResponse.php b/src/applications/transactions/response/PhabricatorApplicationTransactionWarningResponse.php
new file mode 100644
--- /dev/null
+++ b/src/applications/transactions/response/PhabricatorApplicationTransactionWarningResponse.php
@@ -0,0 +1,58 @@
+<?php
+
+final class PhabricatorApplicationTransactionWarningResponse
+  extends AphrontProxyResponse {
+
+  private $viewer;
+  private $exception;
+  private $cancelURI;
+
+  public function setCancelURI($cancel_uri) {
+    $this->cancelURI = $cancel_uri;
+    return $this;
+  }
+
+  public function setException(
+    PhabricatorApplicationTransactionWarningException $exception) {
+    $this->exception = $exception;
+    return $this;
+  }
+
+  protected function buildProxy() {
+    return new AphrontDialogResponse();
+  }
+
+  public function reduceProxyResponse() {
+    $request = $this->getRequest();
+
+    $title = pht('Warning: Unexpected Effects');
+
+    $head = pht(
+      'This is a draft revision that will not publish any notifications '.
+      'until the author requests review.');
+    $tail = pht(
+      'Mentioned or subscribed users will not be notified.');
+
+    $continue = pht('Tell No One');
+
+    $dialog = id(new AphrontDialogView())
+      ->setViewer($request->getViewer())
+      ->setTitle($title);
+
+    $dialog->appendParagraph($head);
+    $dialog->appendParagraph($tail);
+
+    $passthrough = $request->getPassthroughRequestParameters();
+    foreach ($passthrough as $key => $value) {
+      $dialog->addHiddenInput($key, $value);
+    }
+
+    $dialog
+      ->addHiddenInput('editEngine.warnings', 1)
+      ->addSubmitButton($continue)
+      ->addCancelButton($this->cancelURI);
+
+    return $this->getProxy()->setDialog($dialog);
+  }
+
+}