There are some great source level debuggers for the Mac, but all of them are programs that depend on the toolbox to interact with you and draw stuff on the screen to help you figure out what’s going wrong. When your program is really in dire straits, and has taken the operating system down with it, there is no substitute for a debugger that doesn’t need a working operating system. For the pre-OSX Macintosh, this was usually Macsbug. Macsbug was great, because it was so simple. It didn’t draw windows or use the mouse. You could type cryptic 2-letter commands, if you could remember them, and try understand the stuff Macsbug would type back at you. With some luck you might find out how your program killed itself (and maybe the operating system too). I have a couple of books on Macsbug, that I have read over and over, basically just trying to remember the commands. I can remember some stuff, and I have used it some. Despite my best efforts, I could never like Macsbug. Probably, this was because I had seen something that did the same job, but was way better.