Heh... that's why I'm able to make a living. People think that working software won't break. Then, the data reaches a point that every computer has... it's called the "tipping point". That's where working software that has (supossedly) good performance suddenly has very bad performance. It also happens when folks can least afford for it to happen... month end runs, tax time, etc, etc.

Just like a truck, the bearings and suspension have to be able to "carry the load". In software, most people don't anticipate the load. The software runs just fine until one day, the load just gets too big and suddenly that proverbial truck breaks.

Part of software maintenance is to check the performance of the code now and again so that as data grows, you can see if the software is beginning to go non-linear in performance and a few other measurements. For example, if a screen was returning in a half second and it's suddenly returning on a consistent basis in a second, something with the data is likely the problem and it's warning you that you're likely to have a substantial performance problem in the future.

Bottom line is, working code can break... don't let it be a surprise when you can least afford it.