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.