When your software is tightly coupled to some complicated external component, the most likely cause of bugs is bad assumptions about how that component behaves.

When your software is tightly coupled to some complicated external component, the most likely cause of bugs is bad assumptions about how that component behaves. The integration tests are therefore the most important, since those are what will expose the bad assumptions. I wrote a program that used the computer's MAC address. The API that reports the MAC strips leading 0s, but my MAC had no leading 0s and so I I did not notice this behavior during development. If I had just captured and replayed this result, my tests would have passed on all machines, even affected ones. But because my tests exercised the real API, I encountered the bug and was able to work around it.