Two things I would add: 1. Think about any unwarranted assumptions you might be making about context. Is there some environment variable that needs to be set

Two things I would add: 1. Think about any unwarranted assumptions you might be making about context. Is there some environment variable that needs to be set that has become unset or had its value change? Did the OS version change? Some config file? 2. If you can find a working version of whatever it is you're trying to fix, do an incremental search to find the differences between that and the broken version. If everything is in version control this is easy. If not, incrementally change the working version to be more like the broken version until the working version breaks. (This technique is especially useful when dealing with large legacy code bases that you are not intimately familiar with.)