Unexpected use of template {{21}} - see Template:21 for details.
Template documentationTemplates {{1}}, {{2}}, {{3}}, etc. warn readers/editors that a typo exists in the page they are reading/editing. If {{21}} is called, you probably meant the parameter 21 (as {{{21}}} with triple braces), but in fact only double braces were used, invoking {{21}}. In addition to a warning, it also adds pages to Category:Templates with incorrect parameter syntax. For more information on how template parameters work, see mw:Help:Templates#Parameters.
However, the most problems have been caused by omitting the first brace "{" of a parameter and getting {{21}}}, as invoking Template:21 +"}" rather than parameter 21. This can be an insidious problem, especially when the transclusion of Template:21 is hidden, so that the effect is invisible to the person editing a page. For that reason, Template:21 now issues a warning to the user.
With that warning, the prior invisible access to Template:21 can now be understood to be a formerly unseen problem coded within a page.
- Note: this will also pick up several variations such as {{21|}} for {{{21|}}} and {{{21}}.
Periodically, people check what-links-here to see if anyone has accidentally invoked Template:21.