There are bugs that don't appear as such at the unit level. Two objects might individually be in perfectly valid states, but those states can be invalid at a s

There are bugs that don't appear as such at the unit level. Two objects might individually be in perfectly valid states, but those states can be invalid at a system level. Which is why there can be a false sense of security with unit testing. When you see 100% code coverage, it implies that you've tested every possible scenario, but with most non-trivial software, you haven't. Did you test what happens when the methods are passed in 100GB of data?