diff --git a/src/applications/project/__tests__/PhabricatorProjectCoreTestCase.php b/src/applications/project/__tests__/PhabricatorProjectCoreTestCase.php
--- a/src/applications/project/__tests__/PhabricatorProjectCoreTestCase.php
+++ b/src/applications/project/__tests__/PhabricatorProjectCoreTestCase.php
@@ -1044,6 +1044,21 @@
     $this->moveToColumn($user, $board, $task_a, $column, $column, $a_options);
     $new_projects = $this->getTaskProjects($task_a);
     $this->assertEqual($old_projects, $new_projects);
+
+
+    // Add the parent project to the task. This should move it out of the
+    // milestone column and into the parent's backlog.
+    $this->addProjectTags($user, $task, array($board->getPHID()));
+    $expect_columns = array(
+      $backlog->getPHID(),
+    );
+    $this->assertColumns($expect_columns, $user, $board, $task);
+
+    $new_projects = $this->getTaskProjects($task);
+    $expect_projects = array(
+      $board->getPHID(),
+    );
+    $this->assertEqual($expect_projects, $new_projects);
   }
 
   public function testColumnExtendedPolicies() {
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
@@ -677,6 +677,8 @@
         }
 
         $editor->save();
+
+        $this->updateWorkboardColumns($object, $const, $old, $new);
         break;
       case PhabricatorTransactions::TYPE_VIEW_POLICY:
       case PhabricatorTransactions::TYPE_SPACE:
@@ -3603,4 +3605,48 @@
     return true;
   }
 
+  private function updateWorkboardColumns($object, $const, $old, $new) {
+    // If an object is removed from a project, remove it from any proxy
+    // columns for that project. This allows a task which is moved up from a
+    // milestone to the parent to move back into the "Backlog" column on the
+    // parent workboard.
+
+    if ($const != PhabricatorProjectObjectHasProjectEdgeType::EDGECONST) {
+      return;
+    }
+
+    // TODO: This should likely be some future WorkboardInterface.
+    $appears_on_workboards = ($object instanceof ManiphestTask);
+    if (!$appears_on_workboards) {
+      return;
+    }
+
+    $removed_phids = array_keys(array_diff_key($old, $new));
+    if (!$removed_phids) {
+      return;
+    }
+
+    // Find any proxy columns for the removed projects.
+    $proxy_columns = id(new PhabricatorProjectColumnQuery())
+      ->setViewer(PhabricatorUser::getOmnipotentUser())
+      ->withProxyPHIDs($removed_phids)
+      ->execute();
+    if (!$proxy_columns) {
+      return array();
+    }
+
+    $proxy_phids = mpull($proxy_columns, 'getPHID');
+
+    $position_table = new PhabricatorProjectColumnPosition();
+    $conn_w = $position_table->establishConnection('w');
+
+    queryfx(
+      $conn_w,
+      'DELETE FROM %T WHERE objectPHID = %s AND columnPHID IN (%Ls)',
+      $position_table->getTableName(),
+      $object->getPHID(),
+      $proxy_phids);
+  }
+
+
 }