You are not going to need it
When you are writing code it is common to think "I may need XYZ" but I am not really going to need it, Agile advocates using the available time as best as possible and if you do not really need it then why waste time? Agile also advocates refactoring to improve and tune your code and these two concepts fit together, consider very carefully when refactoring what your code does and if you really need said refactoring. Ron puts it best
The best way to implement code quickly is to implement less of it. The best way to have fewer bugs is to implement less code.