The one thing that I didn't see mentioned is the fact that you can test very specific branches of code with higher-level testing, but the same cannot be done in

The one thing that I didn't see mentioned is the fact that you can test very specific branches of code with higher-level testing, but the same cannot be done in reverse. In fact, higher-level tests, especially tests driven with real--world data, routinely execute branches of code in ways that are unforeseen by the developer and unlikely to have been tested at a lower level. I suspect that fuzz testing will some day remove much of the need for lower-level testing.