I solved a performance bug that was probably going to cost my company the majority of our customers if we didn't sort it out as it made the app unusable. It had been going on for months. None of the dev team had any idea. APM was throwing out a bunch of red herrings. I haven't written a line of code in the app or a line of production Java (the codebase language) in my life. In the end I tracked it down to a single line of configuration code. Performance went from being atrocious to being one of the best performing apps I've ever seen.
This was not technically "hard" as in complicated but more so hard in the sense that it was very high stakes and the time pressure was high.
I do this a lot but it wouldn't have helped in this case unfortunately. Also the code base is over 700k lines which added another parameter to the mix.
This was not technically "hard" as in complicated but more so hard in the sense that it was very high stakes and the time pressure was high.