Qbix Calendar (also known as Calendar 2), an alternative to the Calendar application built in to macOS and available on Apple's Mac App Store, had included a cryptocurrency mining process that utilized user's idle system resources in the background. This application was seemingly the only one on the Mac App Store that mined cryptocurrency (specifically Monero) in the background.
Though the developer included an option to disable this background cryptocurrency mining functionality, Ars Technica reported a bug that ensured that the mining background process kept running regardless of if the user disabled the option. There was also a second bug that ensured that the mining process actually utilized more computer resources than was originally intended.
As of this writing, Qbix Calendar (known as Calendar 2 in the Mac App Store, named as such to directly compete with the Apple Calendar included with macOS) has been pulled from the storefront, with the founder of Qbix, Gregory Magarshak, telling Ars Technica that the functionality will be removed in future versions of the application. It's currently unknown if the application's removal was caused by Apple or by Qbix.