That's the title of a blog post I would love to see Jeff Atwood write. As I was sitting there playing with one of my children this evening I noticed we were using a box of Legos which contained a mixture of their brand new ones and those from my childhood (30+ years old in most cases). Suddenly it dawned on me that none of these pieces had any comaptibility issues -- we could mix and match at will and they always, always fit. Of course, there are exceptions when it comes to Duplo and Technic -- but those have always been present. Talk about legacy support...!