It sounds like the category was added by the template. You need to use the noinclude tag on the template page to prevent it from adding the cateogry to pages it is used on. For example:
This is my template.<!--
--><includeonly>[[Category:Uses a dummy template]]</includeonly><!--
--><noinclude>
[[Category:Templates]]
</noinclude>
This example template will add the pages it is used on to the category "Uses a dummy template" however the template page itself won't be added to that category. The template page itself will be added to category "Templates" but pages it is used on won't be.