Software is Concrete. Once poured it becomes extremely difficult and very expensive to change.
It comes from a blog post by Robert Howe, CEO of Verum, a company selling formal-methods-based and model-based programming tools. It does capture something of the phenomenon we all know: that software can be pretty darn hard to change, once it has shipped and is in use. It fits well with the fact that the later bugs are found, the more expensive they are to fix.
But it also provoked quite a bit of opposition when I put the quote up on Facebook, and I have to agree that maybe not all is as simple as that blog makes it out to be.