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

Getting given a system someone else worked on, spending days understanding the system, to identify that at the present rate it'll start to fail inside of a few months, escalating that, being ignored, then seeing the system begin to fail as expected.

It's like watching a train wreck in slow motion, except I am in the train.

Legacy code is always frustrating. I had to pick up Delphi at one point to fix a bug in an application who's only job was to relay specific windows messages (essentially a broker).

5 sold days getting the environment setup (every dependency had to be in the PATH), 20 min to make the change then another 3 days getting the installer build environment setup.

But don't conflate tough with what frustrates people. I spent 2-3 years designing and building 3D CAM software that presented MANY challenging problems from abstracting platform API's to optimzing toolpaths for cost effective manufaturing.

Not once did I get frustrated with the learning or implemention process. The frustration in a sr devs life (in my experience) usually comes from uper management putting "really cool" features on the back-burner to never be seen again.