Recently, Apple sent all developers this email, which contains helpful information in case you suddenly experience problems with applications obtained from the Mac App Store:
As a reminder, the Apple Worldwide Developer Relations Certification Authority Intermediate Certificate expires on February 14, 2016 […].[…], we want to make you aware of potential issues your users may encounter due to the renewed certificate. Users running OS X El Capitan (v10.11 or v10.11.1) may receive a notification that your Mac app is damaged if it utilizes receipt validation to request a new receipt from Apple. They can resolve this issue by restarting their Mac or updating to OS X El Capitan (v10.11.2). Mac App Store customers running OS X Snow Leopard (v10.6.8) will be unable to purchase new apps or run previously purchased apps that utilize receipt validation until they install the Mac App Store Update for OS X Snow Leopard, which will soon be available via OS X Software Update.