archive.php showing posts from wrong categories

I’m using a customized archive.php script for my custom theme, but basically without any apparent & significant big change. Header, loop & footer, without altering the loop, so there’s no custom query.

The problem is that when the script is used to show posts belonging to one category, some posts belonging to other categories are shown.

How is that?

Details: I do use ‘posts_where’ filter to filter out particular posts titles, however this should not interfere with categories, particularly because the filter uses “AND” condition on post_title.