The hardest code for me to debug in general seems to be JavaScript. For the longest time there has been no "serious" debugging environment for it and although Chrome has made tremendous strides here, there still isn't the debugging capability that could rival GDB, OllyDbg, and WinDbg for example.
I also thought that trying to debug code which was crashing the browser by using a tool in that same browser was utterly insane. Yeah, I don't miss JS :D