English: A page is added to this category if a template that invokes Module:Countries is used with a parameter regarded as invalid by the module.

• To know which invalid parameters are used, edit the page and preview it to see strong red warnings showing explicitly the incorrect parameter name and its value (these additional error messages are displayed only in this preview mode; when the page is saved and viewed it is only categorized below if there's still problems, but no warning is dispayed).
• The most frequent errors are caused by omitting the explicit parameter names (notably |prefix= for the first parameter: there are no positional parameters because all parameters are optional and this could be specifying another parameter), or by omitting the required = sign before the value or incorrectly replacing it with other characters such as a : (creating an additional positional parameter also ambiguous), or by some typo in the supported parameter names (such as |sufix= instead of |suffix=), or by leaving a trailing | in the parameter list (which adds a dummy positional parameter).