Mage registry key "_singleton/tax/observer" already exists - php

Question

I have encountered this error as I was doing this.
The error happen in both frontend and backend.
I have tried below action:
Flushing the cache (delete everything in var/cache folder) : doesn't work.
Went to the compiler and noticed it was disabled
Tried & running the compiler : also doesn't work
Any suggestions?
Trace:
#0 /var/www/vhosts/jomgroupon.com/httpdocs/app/Mage.php(222): Mage::throwException('Mage registry k...')
#1 /var/www/vhosts/jomgroupon.com/httpdocs/app/Mage.php(476): Mage::register('_singleton/tax/...', false)
#2 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Model/App.php(1316): Mage::getSingleton('tax/observer')
#3 /var/www/vhosts/jomgroupon.com/httpdocs/app/Mage.php(447): Mage_Core_Model_App->dispatchEvent('catalog_product...', Array)
#4 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Catalog/Model/Resource/Product/Collection.php(526): Mage::dispatchEvent('catalog_product...', Array)
#5 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Eav/Model/Entity/Collection/Abstract.php(885): Mage_Catalog_Model_Resource_Product_Collection->_afterLoad()
#6 /var/www/vhosts/jomgroupon.com/httpdocs/lib/Varien/Data/Collection.php(301): Mage_Eav_Model_Entity_Collection_Abstract->load()
#7 /var/www/vhosts/jomgroupon.com/httpdocs/app/design/frontend/base/default/template/catalog/product/widget/new/column/new_default_list.phtml(36): Varien_Data_Collection->getItems()
#8 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Block/Template.php(241): include('/var/www/vhosts...')
#9 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Block/Template.php(272): Mage_Core_Block_Template->fetchView('frontend/base/d...')
#10 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Block/Template.php(286): Mage_Core_Block_Template->renderView()
#11 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Block/Abstract.php(863): Mage_Core_Block_Template->_toHtml()
#12 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Block/Text/List.php(43): Mage_Core_Block_Abstract->toHtml()
#13 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Block/Abstract.php(863): Mage_Core_Block_Text_List->_toHtml()
#14 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Block/Abstract.php(582): Mage_Core_Block_Abstract->toHtml()
#15 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Block/Abstract.php(526): Mage_Core_Block_Abstract->_getChildHtml('right', true)
#16 /var/www/vhosts/jomgroupon.com/httpdocs/app/design/frontend/default/skytheme/template/page/2columns-right.phtml(58): Mage_Core_Block_Abstract->getChildHtml('right')
#17 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Block/Template.php(241): include('/var/www/vhosts...')
#18 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Block/Template.php(272): Mage_Core_Block_Template->fetchView('frontend/defaul...')
#19 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Block/Template.php(286): Mage_Core_Block_Template->renderView()
#20 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Block/Abstract.php(863): Mage_Core_Block_Template->_toHtml()
#21 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Model/Layout.php(555): Mage_Core_Block_Abstract->toHtml()
#22 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Controller/Varien/Action.php(390): Mage_Core_Model_Layout->getOutput()
#23 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/local/Gclone/Deal/controllers/IndexController.php(72): Mage_Core_Controller_Varien_Action->renderLayout()
#24 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Controller/Varien/Action.php(419): Gclone_Deal_IndexController->viewAction()
#25 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Controller/Varien/Router/Standard.php(250): Mage_Core_Controller_Varien_Action->dispatch('view')
#26 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Controller/Varien/Front.php(176): Mage_Core_Controller_Varien_Router_Standard->match(Object(Mage_Core_Controller_Request_Http))
#27 /var/www/vhosts/jomgroupon.com/httpdocs/app/code/core/Mage/Core/Model/App.php(354): Mage_Core_Controller_Varien_Front->dispatch()
#28 /var/www/vhosts/jomgroupon.com/httpdocs/app/Mage.php(683): Mage_Core_Model_App->run(Array)
#29 /var/www/vhosts/jomgroupon.com/httpdocs/index.php(87): Mage::run('', 'store')
#30 {main}

Answer 1

Check this file: /var/www/vhosts/jomgroupon.com/httpdocs/app/code/local/Gclone/Deal/controllers/IndexController.php if you have call to the Mage::register() method
Mage::register('_singleton/tax/observer', $theObject)
Then change that call with adding third parameter as true:
Mage::register('_singleton/tax/observer', $theObject, true);
For further explanation please see: Error processing request: Mage registry key "_singleton/callforprice/observer" already exists

Answer 2

Finally, I found the solution here:
1st: looks tax/observer does not exists in your site, check if Observer.php is in app/code/core/Tax/Model folder
2nd: check if affiliateplusprogram/observer class file is there.
The found observer file names have been changed wrongly in both folders. Once corrected, the error no longer prompted up when clicking checkout