A great recent example is the ugly Rube Goldberg machine under the OSX GUI that led to the recent root bug. Look under the hood and it's really a mess. Windows

A great recent example is the ugly Rube Goldberg machine under the OSX GUI that led to the recent root bug. Look under the hood and it's really a mess. Windows is even worse. It's amazing it works at all. Linux is generally cleaner in the auth department but has many other abominations. The only clean software is either software nobody uses or software whose scope is rigorously limited. The latter is very very hard to defend when people are using it. Everyone wants it to do some new thing or be compatible with some other system.