Principle that states that small programs are well designed, but as soon as the programs grow bigger, they become increasingly harder to understand. Eventually, they become unmaintainable.
This is why "let's just add one more feature" is bad way of making programs.
Instead of building on structures what were not intended to support features you need, you should review the design first to avoid the "taste of kluges". One solution is to do refactoring over time.
See: bloat