Modifying a binary (we did not have the source) to work on a different CPU architecture. That was my most challenging problem.

What makes me frustrated? Inconsistent interfaces, uncessisary abstraction, premature optimisation and juniors and intermediates who don’t profile before attempting optimisation

Though these things don’t really “frustrate” me - as you get older your time horizons broaden and everything is in perspective, and often I just fix these problems myself, it’s quite enjoyable fixing these things, like a form of meditation