Me, with zero C/C++ experience, being asked to figure out why the newer version of the Linux kernel is randomly crash-panicking after getting cross-compiled for a custom hardware box.
("He's familiar with the the build-system scripts, so he can see what changed.")
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I spent weeks of testing slightly different code-versions, different compile settings, different kconfig options, knocking out particular drivers, waiting for recompiles and walking back and forth to reboot the machine, and generally puzzling over extremely obscure and shifting error traces... And guess what? The new kernel was fine.
What was not fine were some long-standing hexadecimal arguments to the hypervisor, which had been memory-corrupting a spot in all kernels we'd ever loaded. It just happened to be that the newer compiles shifted bytes around so that something very important was in the blast zone.
Anyway, that's how 3 weeks of frustrating work can turn into a 2-character change.