From Arnold Kling’s Grover and the Silver Bullet:
When my kids were younger, one of my favorite bedtime stories for them was a Sesame Street book called “Grover?s Resting Places.” The most adorable scene is where Grover starts rummaging through a toy box looking for something. After he has strewn most of the contents of the box all over his room, he pauses and says, “Uh-oh. I had better be careful, or I will make a mess.”
Software almost always has a similar phase in the actual development lifecycle. Some time after the system has been in production, the business needs will tend to evolve in such a way that developers are forced to stretch it and modify it, piling up ad hoc code. Then, like Grover, the developers may pause and raise a concern that they might create a mess — when in fact the mess already is at hand.