Unexpected use of template {{15}} - see Template:15 for details.
Template documentationTemplates {{1}}, {{2}}, {{3}}, etc. warn readers/editors that a typo exists in the page they are reading/editing. If {{15}} is called, you probably meant the parameter 15 (as {{{15}}} with triple braces), but in fact only double braces were used, invoking {{15}}. 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 {{15}}}, as invoking Template:15 +"}" rather than parameter 15. This can be an insidious problem, especially when the transclusion of Template:15 is hidden, so that the effect is invisible to the person editing a page. For that reason, Template:15 now issues a warning to the user.
With that warning, the prior invisible access to Template:15 can now be understood to be a formerly unseen problem coded within a page.
- Note: this will also pick up several variations such as {{15|}} for {{{15|}}} and {{{15}}.
Periodically, people check what-links-here to see if anyone has accidentally invoked Template:15.