If you want to add a warning message on a substitution-only template, when the template has not been substituted, to substitute it, you can do so through the following:

<includeonly>{{subst:</includeonly><includeonly>null|<span style="font-size: large; color: red">'''Have to subst this template.  Replace {{templatenamehere}} with {{subst:templatenamehere}}'''</span>}}</includeonly>

This leaves an ugly line of red-coloured large-sized text stating that you forgot to subst the template "templatenamehere" (replace that string with the name of your template)

This implementation requires the existence of template:null on your wiki of the same implementation as that found at w:c:templates:template:null, so copy that to your wiki. Alternately, you can copy wikipedia:en:template:void instead, and replace the instance of "null" with "void".

If you need instruction on how to copy that template off w:c:templates, please read w:c:templates:Help:How to copy templates to your wiki

ForestMonthZero 10:00, December 21, 2011 (UTC)