My customer has a big main code line and one external reference. I branched from the HEAD of TRUNK and my working copy was pointing to the branch. Really, I swear
Now I made some changes and wanted to commit, and to my surprise I got “you don’t supposed to commit on tag” kind of message. I guess, a friendly SVN trigger left by someone prudent. Why and what should I do?

I am on the branch:

> svn info
Path: .
URL: https://my.host/svn/main/branches/MY_CURRENT_BRANCH
...

But…

By typing svn propget -R svn:externals . you can check the externals:

> svn propget -R svn:externals .
. - res  https://my.host/svn/intern/tags/MY_COOL_TAG

 
So, it appears that my res directory can't be committed now! It points to TAG.

What to do:

1

Make sure external points to the BRANCH or HEAD

In my example, res -> https://my.host/svn/intern/tags/MY_COOL_TAG.
What I can and probably want to do is to take the HEAD of https://my.host/svn/intern/trunk and branch it.

Now, I just do the switch on res directory. But wait, what about my changes? That is not a problem!

Remember, that switch is actually a form of update so the changes remain.

Now, I have

> svn propget -R svn:externals .
. - res  https://my.host/svn/intern/branches/FOR_MY_FEATURE

 
and I also have all my feature implemented.

2

Commit or reintegrate

Now you can commit all you have done to the external location. That part is trivial.
Done! Have fun!