Templates {{1}}, {{2}}, {{3}}, etc. warn readers/editors that a typo exists in the page they are reading/editing. If {{11}} is called, you probably meant the parameter 11 (as {{{11}}} with triple braces), but in fact only double braces were used, invoking {{11}}. 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 {{11}}}, as invoking Template:11 +"}" rather than parameter 11. This can be an insidious problem, especially when the transclusion of Template:11 is hidden, so that the effect is invisible to the person editing a page. For that reason, Template:11 now issues a warning to the user.
With that warning, the prior invisible access to Template:11 can now be understood to be a formerly unseen problem coded within a page.