It is my impression that whenever a feature is added, there may be many things
which are
affected by adding that feature. Like if I add a new aspect to the data model.
Then I add
code to save that data. To load that data. Create a form to present that data.
Add some
logic which keys on values of that data. Then you must deal with old saved
models and
update them. Except in some cases the correct update is not clear so a
not-initialized
option is put in place. So cases where this data is accessed now needs to check
for this
condition and properly handle these exceptional cases.

And by not considering those things, you introduce bugs into the software. If
you consider
some of these, but forget others, the program remains broken. TDD helps avoid
bugs by
telling you when the code you changed breaks existing functionality. However, I
don't see
how to avoid breaking the code over new behaviour. I don't see a way to
properly identify
what new behaviour will need to be covered. I want to be sure I'm not
forgetting cases, but
currently I'm overlooking too many of these effects.

What's the path to enlightenment?