How are regressions possible?

[Jonathan Wakely]

On Sun, 2 Sep 2018, 14:46 Jeff Donner, <jeffrey.donner@gmail.com> wrote:

> Hi - I thought bugs were added to the test suite, and so, regressions
> should never occur. How is this understanding wrong and why isn't it this
> way?
>

Not all regressions are recurring bugs that have already been fixed.
Anything that used to work and then stops working is a regression. If the
thing that worked wasn't covered by any test in the testsuite the
regression goes unnoticed.