Did anyone encounter a problem with a third party checkout extension like Amasty OPC or IWD OPC? I haven't upgraded yet to Magento 1.9.4.2 because I read in the release notes that:
"Third-party checkout extensions and closed security cases will either not work securely or will not work at all."
https://devdocs.magento.com/guides/m1x/ce19-ee114/ce1.9_release-notes.html#ce19-1942