Apple did some pretty brilliant stuff with testing. E.g. they had it silently test install itself on random user devices, verify everything was fine, and revert

Apple did some pretty brilliant stuff with testing. E.g. they had it silently test install itself on random user devices, verify everything was fine, and revert for several versions prior to release (presumably they picked users with sufficient spare room). The actual rollout was stunningly issue-free. As an aside, this is one of the several unbelievably slick updates Apple has pulled off in its history, including: - the switch to 32-bit clean addresses only (which was probably the most painful switch Apple ever did!) in System 7.6 - the switch to PowerPC (which caused fewer problems than the 32-bit clean switch!) - the switch to Mac OS X - the switch to Intel - the switch to 64-bit on desktop - the switch to 64-bit on iOS Microsoft, (in most cases rightly) vaunted for backwards compatibility has had horrific snafus such as DOS 5 being the first version of DOS whose RESTORE program could read BACKUP files from the previous version of DOS. (It's also worth noting that NeXT itself managed to support four runtime architectures with "quad fat binaries".)