In this article we'll list links to different Magento2 github issues listing issues in different Magento 2 versions that affect checkout or Magento 2 related to checkout directly and are out of scope of our extension:

• Exception in frontend: FastCGI sent in stderr: "PHP message: PHP Fatal error: Uncaught TypeError: Argument 1 passed to Magento\Quote\Model\Cart\Totals::setExtensionAttributes() must be an instance of Magento\Quote\Api\Data\TotalsExtensionInterface, instance of Magento\Quote\Api\Data\AddressExtension given ... https://github.com/magento/magento2/issues/12655
• Exceptionin backend: Exception #0 (Exception): Notice: Array to string conversion in /.../vendor/magento/module-shipping/Model/Config/Source/Allmethods.php on line 61 ... https://github.com/magento/magento2/issues/13136
• Ajax call to set-payment-methods fails with validation message on frontend 
  https://github.com/magento/magento2/issues/28161 solution is to move mentioned file to local theme scope and comment out the ajax call that fires after a ajax call :) 
• Region field is not hidden when set to be invisible and optional from admin (magento 2.4.0, 2.4.1)  https://github.com/magento/magento2/issues/30747 and temporary solution is to apply this in theme context https://github.com/magento/magento2/issues/30747#issuecomment-720412731
• Magento 2.4.3 Vertex_AddressValidation breaks checkout on billing-validation-mixin.js #33815 (https://github.com/magento/magento2/issues/33815) due calling address template rendering too early and even when extension is not configured at all , solution is to disable this extension when not used , rewrite the mixin if needed to be used (Support can assist) 
• Magento 2.4.3  JS error in console on checkout when recaptcha for checkout/placing order is not enabled #33741 (https://github.com/magento/magento2/issues/33741) due including all the captcha resources even when disabled . Solution is to disable unused recaptcha extensions completely or wait for patches