Speaking from experience, I've had to worry about what implementation of sort is being used in many languages, from Java to C++, even Go and Python.

There are a lot of details to get right: how are elements compared? Is the sort stable? Is it efficient for small N? Is it efficient for nearly-sorted arrays? Is it efficient when almost all the elements compare equal? Is it guaranteed O(n log n) or average? If average, is there an input that reliably triggers n^2 behavior making it a DDoS vector?

Anything is a leaky abstraction when you care enough.