Since a couple of weeks, I have a notification in the App Store that I need to update Mountain Lion to 10.8.5. But I'm on Mojave 10.14.6. Does anyone know why and do you know what will happen if I click update?
The logical supposition would be that you probably have the Mountain Lion installer somewhere on the drive and you are getting a notification to update it.
But App Store is full of bugs across iOS and macOS so everything in possible