After requirements, design, and build followed unit, system, and user acceptance testing and then assuming all of the runtime kinks, syntax errors, bad configuration parameters, etc. have been sussed out of a program, it is finally ready to run in a production environment.
And it’s at that point, that things will eventually go awry. You see, the production environment is the real world. Your program was built to accommodate specific use-cases, but you (and your customer) can only guess so far ahead into the future. New and unexpected conditions for your code to deal with are already on the way.
Perhaps a data input is formatted incorrectly, or has special characters that break your parser, or there’s simply too much of it. Maybe your program becomes more popular than expected and its use exceeds the original design constraints, meaning there’s a risk of exceeding available disk space, RAM, quotas, whatever. Users always tease out new bugs, as they tend to explore and use your program in novel ways that you didn’t anticipate.
Point is, a bit of code doesn’t “wear out” or “break down”. It executes the same every time. But the surrounding environment that supports the program, feeds it, accommodates the results, and all that is always growing, in flux, and will likely evolve beyond what your program was originally coded to handle.