We got new problem with one of our site at the checkout. We are using paypal standard and when we are ready to be redirected to paypayl we got bounced back to our cart.
We got this error : Fatal error: Undefined class constant 'CHECK_USE_CHECKOUT' in/home/hobbysou/public_html/app/code/core/Mage/Checkout/Model/Type/Onepage.php on line625
@patricebois this means CHECK_USE_CHECKOUT couldn't find Constance. Magento has introduce Constance in 1.8 version and seems you're using 1.7 version...
Have been trying to upgrade your version ? 1.7 to 1.8 ? I suggest you checking the difference between class. follow below file for the same.