In the error log, all I see is a warning. Doesn't seem like this should result in failing to save, though.
Code:
#0 src/addons/vw/vw/_core/instance/parser/html/vw.php(719): XF::handlePhpError(2, '[E_WARNING] Und...', '/home/badgerand...', 719)
#1 src/addons/vw/vw/_core/instance/parser/html/vw.php(706): vw_Parser_Html_Instance->plain_tag(Array, 1)
#2 src/addons/vw/vw/_core/model/parser/sideways/template/xf0.php(1142): vw_Parser_Html_Instance->plain_text(Array, 1)
#3 src/addons/vw/vw/_core/model/parser/sideways/template/xf0.php(481): vw_Parser_Sideways_Template_Model_XF0->process_params('sotd', '|-
| __P__(Them...', Array, Array, Array)
#4 src/addons/vw/vw/_core/model/parser/sideways/template/xf2.php(72): vw_Parser_Sideways_Template_Model_XF0->process_replacements('<!-- VW:TEMPLAT...', Array, false)
#5 src/addons/vw/vw/_core/model/parser/handle/base/vw.php(172): vw_Parser_Sideways_Template_Model_XF2->process_replacements('<!-- VW:TEMPLAT...', Array)
#6 src/addons/vw/vw/_core/model/parser/handle/wrapper/vw.php(213): vw_Parser_Handle_Base_Model->handle_bbcode_template(Array)
#7 src/XF/BbCode/Renderer/Html.php(310): vw_Parser_Handle_Wrapper_Model->vwcode_wrapper(Array, NULL, Array, Array, Object(vw\vw\XF\BbCode\Renderer\Html))
#8 src/addons/vw/vw/XF/BbCode/Renderer/Html.php(523): XF\BbCode\Renderer\Html->renderTag(Array, Array)
#9 src/XF/BbCode/Traverser.php(63): vw\vw\XF\BbCode\Renderer\Html->renderTag(Array, Array)
#10 src/addons/vw/vw/XF/BbCode/Renderer/Html.php(553): XF\BbCode\Traverser->renderSubTree(Array, Array)
#11 src/XF/BbCode/Traverser.php(39): vw\vw\XF\BbCode\Renderer\Html->renderSubTree(Array, Array)
#12 src/addons/vw/vw/XF/BbCode/Renderer/Html.php(154): XF\BbCode\Traverser->renderAst(Array, Object(SV\UserMentionsImprovements\XF\BbCode\RuleSet), Array)
#13 src/XF/BbCode/Traverser.php(22): vw\vw\XF\BbCode\Renderer\Html->renderAst(Array, Object(SV\UserMentionsImprovements\XF\BbCode\RuleSet), Array)
#14 src/addons/vw/vw/XF/BbCode/Renderer/Html.php(149): XF\BbCode\Traverser->render('[h=2]Introducti...', Object(vw\vw\XF\BbCode\Parser), Object(SV\UserMentionsImprovements\XF\BbCode\RuleSet), Array)
#15 src/addons/vw/vw/XF/BbCode/Renderer/Html.php(85): vw\vw\XF\BbCode\Renderer\Html->vwParentRender('[h=2]Introducti...', Object(vw\vw\XF\BbCode\Parser), Object(SV\UserMentionsImprovements\XF\BbCode\RuleSet), Array)
#16 src/addons/vw/vw/_core/instance/parser/renderer/xf2.php(385): vw\vw\XF\BbCode\Renderer\Html->vwRender('[h=2]Introducti...', Object(vw\vw\XF\BbCode\Parser), Object(SV\UserMentionsImprovements\XF\BbCode\RuleSet), Array)
#17 src/addons/vw/vw/_core/model/parser/instance/xf2.php(219): vw_Parser_Renderer_Instance_XF2->render('[h=2]Introducti...')
#18 src/addons/vw/vw/_core/controller/dm/page/vw.php(1370): vw_Parser_Instance_Model_XF2->parse(Array)
#19 src/addons/vw/vw/_core/controller/dm/page/vw.php(951): vw_DM_Page_Controller->process_links()
#20 src/addons/vw/vw/_core/controller/dm/page/vw.php(803): vw_DM_Page_Controller->process_revision()
#21 src/addons/vw/vw/XF/Entity/EntityBase.php(513): vw_DM_Page_Controller->pre_save(true)
#22 src/XF/Mvc/Entity/Entity.php(1359): vw\vw\XF\Entity\EntityBase->_preSave()
#23 src/addons/vw/vw/XF/Entity/EntityBase.php(504): XF\Mvc\Entity\Entity->preSave()
#24 src/addons/vw/vw/_core/controller/dm/xf2.php(367): vw\vw\XF\Entity\EntityBase->vwPreSave()
#25 src/addons/vw/vw/_core/controller/ui/edit/page/vw.php(601): vw_DM_Controller_XF2->pre_save()
#26 src/addons/vw/vw/_core/controller/ui/edit/vw.php(172): vw_UI_Edit_Page_Controller->start_dm(Object(vw_UI_Page_Controller), Array, Array)
#27 src/addons/vw/vw/_core/controller/ui/page/vw.php(474): vw_UI_Edit_Controller->save(Array)
#28 src/addons/vw/vw/_core/controller/ui/page/vw.php(170): vw_UI_Page_Controller->save()
#29 src/addons/vw/vw/_core/controller/ui/stack/vw.php(295): vw_UI_Page_Controller->execute()
#30 src/addons/vw/vw/Pub/Controller/Wiki/Wiki.php(80): vw_UI_Stack_Controller->execute()
#31 src/addons/vw/vw/Mvc/Dispatcher.php(30): vw\vw\Pub\Controller\Wiki\Wiki->_actionIndex(Object(XF\Mvc\ParameterBag))
#32 src/addons/vw/vw/Pub/Controller/Wiki/AbstractController.php(26): vw\vw\Mvc\Dispatcher::dispatch(Object(vw\vw\Pub\Controller\Wiki\Wiki), 'index', Object(XF\Mvc\ParameterBag))
#33 src/XF/Mvc/Dispatcher.php(352): vw\vw\Pub\Controller\Wiki\AbstractController->actionIndex(Object(XF\Mvc\ParameterBag))
#34 src/XF/Mvc/Dispatcher.php(259): XF\Mvc\Dispatcher->dispatchClass('\\vw\\vw\\Pub\\Cont...', 'Index', Object(XF\Mvc\RouteMatch), Object(vw\vw\Pub\Controller\Wiki\Wiki), NULL)
#35 src/XF/Mvc/Dispatcher.php(115): XF\Mvc\Dispatcher->dispatchFromMatch(Object(XF\Mvc\RouteMatch), Object(vw\vw\Pub\Controller\Wiki\Wiki), NULL)
#36 src/XF/Mvc/Dispatcher.php(57): XF\Mvc\Dispatcher->dispatchLoop(Object(XF\Mvc\RouteMatch))
#37 src/XF/App.php(2351): XF\Mvc\Dispatcher->run()
#38 src/XF.php(517): XF\App->run()
#39 index.php(20): XF::runApp('XF\\Pub\\App')
#40 {main}