Ripple effect of changing fields for user profiles & tangent thoughts

Here is a little story to add to the "Lessons Learned" chapter of my Drupal life...

I got an urgent message via Skype:  THE REGISTRATION FORM IS GONE!  Dang, I just saw it yesterday!

On one of my sites I'm using a custom registration form and rendering particular fields. As it turns out, I'm using field groups and had these fields in a group and a sub group. The rendered fields were preceeded by  both groups in the custom form. By changing the way the fields were organized in Manage Fields they were no longer in both of the groups. The custom form didn't like the extra group and could not be called any more. I was getting this error:

  Warning: include() [function.include]: Failed opening '/home/website/public_html/sites/all/themes/website/templates/user-register.tpl.php' for inclusion (include_path='.:/usr/lib/php:/usr/local/lib/php') in theme_render_template() (line 1525 of /home/website/public_html/includes/theme.inc).

with no more explanation.

Once I realized that it may be because the groups were changed, and I deleteded the group that had been removed, then the included template worked and the custom form showed up!
so this:

print drupal_render($form['additional_settings']['group_basic']['group_basicinfo']['field_first_name']);

became:

print drupal_render($form['additional_settings']['group_basic']['field_first_name']);

The fact that this might be a problem didn't even occur to me since I was working on the "user profile edit" and not the "user registration" - not even connecting the dots to the fact that ultimately they are one and the same.

So this now leads me to the thought of "had I not known what happened with the Account Settings would I have been able to figure this out?" Although in this case I am the chief cook and bottle washer for this site It still took me a while to figure it out. If this had been a team situation and one person was working on frontend stuff and one on backend stuff there is a good possibility that finding a solution to this could drag on for quite some time. What is the take away? What could have been a preventative measure in a team situation? What parameters for documenation would have been needed to cover this situation? Or, are there just some things that will end up in MysteryLand? Frontend testing after updates would let me know all is well, and at lest I could rule out the updates as the culprit. A documentation wiki with a notation of the customized registration form would be helpful. Data visualization map for the site?  Ultimately I switched my 'mindhat' from frontend guy to backend guy. This helped me look at the template with a different perspective. Now, that whole concept deserves a post of its own!   :)

Just some thoughts on the 'hiccup du jour"...