It's easy to make the assumption that developers haven't ran the code at all, but I find in reality that's rarely the case. In my experience it almost always has something to do with an unexpected environment variant.
It's very easy as a developer to get so comfortable with the "happy path" of navigating code that you forget to look outside of the happy path when devising your test cases. It takes time to learn to identify the test cases you're not regularly exposing yourself, and these test cases are often ones not described in a requirements or specs doc.
It's very easy as a developer to get so comfortable with the "happy path" of navigating code that you forget to look outside of the happy path when devising your test cases. It takes time to learn to identify the test cases you're not regularly exposing yourself, and these test cases are often ones not described in a requirements or specs doc.