At the risk of sounding uncool, if I was stuck I would flow-chart the gosh-darned thing. I once did this for an engineering subsystem with more than 10,000 lin

At the risk of sounding uncool, if I was stuck I would flow-chart the gosh-darned thing. I once did this for an engineering subsystem with more than 10,000 lines of FORTRAN. It took me more than a month. We were relocating a flight simulator from Italy to Spain where it would go through testing and customer acceptance before being put back in service. By the time I was done, I could debug the system almost as if I had written it myself. It made me look like a freaking genius. The point is _not_ about flowcharts being wonderful. It just forces you to trace the path of execution through the code, and figure out what each bit is trying to do. The tangible documentation (i.e.: stack of flowcharts) that you end up with is great for building your confidence, and convincing your boss that progress has been made.