[ Apologies in advance for not distilling this to a minimum example.  I'm not sure how to do so without a bunch of edits in production pages, and I can't immediately duplicate it in my sandbox. Since I can't figure out how two pieces of the puzzle are interacting, I don't want to blindly drop out something that might be related. ]

I have a template https://en.wikibooks.org/wiki/Template:OpenSCAD_Tutorial/example that attempts to encapsulate the layout for an example.

It's not terribly complex:

<table class="wikitable" style="border: 2px solid Gold;"> <caption style="text-align: left;">[[{{BOOKNAME}}/examples/{{{name}}}|Code&nbsp;-&nbsp;{{{name}}}.scad]]</caption> <tr><td> {{Pre|{{{content|{{msgnw::{{BOOKNAME}}/examples/{{{name}}}}}}}}}} </td></tr> </table> [[File:{{{name}}}.{{{imgtype|jpg}}}|{{{size|680px}}}]] <noinclude>{{BookCat}}</noinclude>

The intent is that it's invoked like so:

{{OpenSCAD Tutorial/example|name=a_small_cube}}

to suck in example source text and an example image from separate pages.

It can also be invoked as:

{{OpenSCAD Tutorial/example|name=a_small_cube|content= cube(10); }}

to specify the source text in-line.[*]

[*] Yes, I know the header still tries to link to the separate page.  It's still a work in progress.

When I first put it together with the separate-page style, everything seemed OK.  Then somebody wrapped the {{BookCat}} in <noinclude>, and "{{BookCat}}" started appearing on the final pages.  I'm not completely sure how <noinclude> works, but OK, I can believe that.  I started a conversation with the other editor about why they added the <noinclude>.

Meanwhile, other editors had commented that the example-in-a-separate-page scheme would be unwieldy.  OK, so I went to back off to the content= style.

And the BookCat problem went away.  The BookCat problem is there for the example-in-separate-page invocation, but not for the in-line invocation.

You can see this at https://en.wikibooks.org/w/index.php?title=OpenSCAD_Tutorial/Chapter_1&o... .  The first example (a_small_cube) uses the in-line style and does not show {{BookCat}}.  The second example (a_cube_with_different_side_lengths) uses the separate-page style and shows the {{BookCat}}.

I can't see how those two things can possibly be tied together.