Isn't end to end already tested when you do different levels of functional tests? For example: 1. Test A -> B (but to do this, need to do x -> z) 2. Test C -> F (but to do this, need to do A -> B) 3. ...
I found a defect in manual end-to-end tests. We were able with developers to isolate the possible root cause in one of the components. We wrote a unit test that reproduces the defect at unit level. ...