The worst I ever had to do was on an embedded machine with no CPU cache. We were getting a crash, but we couldn't figure out how the code was getting into that function. So we hooked up a logic analyzer to the address bus (this was years ago, when CPUs were DIP packages). We wrote to an unused address decode when we entered the function, used that decode to trigger the logic analyzer, and were able to read back what addresses the CPU had been executing before it entered the function. With that information, everything became clear.