I just dreamed up this example to explain why our mistakes show up at system boundaries.

Have you ever noticed that the moon looks much bigger when it is close to the horizon? Well, it turns out this is an optical illusion. The moon is exactly the same size – no further or closer to us on Earth than it would be at a different position in the sky.

The difference is that when the moon is low we can compare its size with objects on the horizon. When it is high there are no nearby reference points for comparison. It just is.

And so to our mistakes. We can be working away, building a brick wall, working on a website, whatever and know of no error in our work. That is until we reach a boundary, and interface. Only then can we find our point of reference.

What’s your point of reference? How can you use it to check what you are doing?