The distribution of when bugs are caught, and what bugs are caught, by different kinds of tests is very different. Units tests catch bugs while they are being w

The distribution of when bugs are caught, and what bugs are caught, by different kinds of tests is very different. Units tests catch bugs while they are being written and when the units they test are being refactored. The bugs they catch are trivial, silly, 'would have been caught anyway'. Outside of those activities, they rarely catch bugs. On the other hand, integration tests more consistently catch bugs and catch bugs you didn't foresee, have to investigate before understanding. You don't remember the last time a unit test caught a bug, because on average it is much longer ago and if it was a test for a unit you wrote this morning, you usually blame it on 'work in progress' and think you would have caught that trivial bug anyway. I think the nature of units tests make it easy to underappreciate them.