Also, if you can do this, then when I haven't added a section heading anywhere, could you make the error a little more intrusive? I had to go back and fix two or three such messages on the same afternoon recently.

Currently the validation is instant and all done in the user's browser. In order to validate based on the contents of a template, the validation would be blocked on a network request, which can take an arbitrarily long time.

Combined with the rarity of this case, and that it already works (just not as smoothly as we'd like), I can't imagine this would be a priority to work on any time soon.

I think this is the same as T295509: Don't ask me to put a section heading in the section heading field if I'm typing it in my new section.