1. Debugging a non-deterministic crash in a huge and complex C++ app. It took me a few weeks of super-focused hard work (kudos to my then team & management at S

1. Debugging a non-deterministic crash in a huge and complex C++ app. It took me a few weeks of super-focused hard work (kudos to my then team & management at Sabre office in Kraków for giving me this time, and understanding and appreciating what I was doing) to solve, with lots of low level debugging down to raw disassembly, as well as code reading and intense thinking and thinking and thinking and thinking. Eventually I managed to narrow it down to a memory race in advanced template code. Some time later I realized it scarred me to C++ (my first love in programming) for life, also opening my eyes to what horror Undefined Behaviors in C++ mean and how they loom ominously over every single seemingly simple character of C++ source code anyone writes. I'm now working in Go and appreciate it so much, as well as Rust etc. It also was one of things that made me appreciate what good management can mean to an employee. 2. Debugging Go runtime in pre-1.0 era, to find out why it was not working stably on Windows. It involved a lot of digging through Go internals (also in Go's old C compiler & assembly), as well as WinDbg disassembly-level debugging. I eventually found out that the Windows call convention was not preserved in some aspect, in that one of the registers was being mangled on return. I see it as my most important contribution to Go, though formally I was not listed as a Contributor through it, though I got a Thank You mention in a commit message I think. But some time later I did some much simpler contribution which got me a then coveted by me (and still valued) entry on the Go contributors list.