I think he means something like the difficulties shown by https://en.wikipedia.org/wiki/Weierstrass_function https://en.wikipedia.org/wiki/Weierstrass_function

I think he means something like the difficulties shown by https://en.wikipedia.org/wiki/Weierstrass_function https://en.wikipedia.org/wiki/Weierstrass_function . There are a lot of conditions a function must satisfy to be differentiable.