A fairly standard question on quora: Why are software development task estimations regularly off by a factor of 2-3? has sparked some discussion.
The best answer given was an extremely entertaining hiking analogy (it's the top-voted answer - go see), explaining the software development process in terms of the fractal-nature of a coastline, with unexpected delays along the way.
I think it's a great analogy as it goes and would be extremely helpful in getting the idea of unexpected scope-creep across to a complete IT-newbie.
The answer then spawned a plethora of comments and even a counter-post that is also really interesting: Why Software Development Estimations Are Regularly Off, which explains that the hiking analogy is way off, and that software estimation is more like inventing.
I agree... though I still see great value in the hiking analogy for explaining "what goes wrong" on the kind of project that encounters the problems described.
The counter-post has itself sparked a discussion on Y-combinator which goes into a lot more detail about estimation issues in general.
It's all provided me with an entertaining read on an otherwise well-picked-over subject.