Problem with media query breakpoints in Twenty Twelve

according to Firefox’ web developer add-on, the ‘responsive-twentyten’ plugin seems to get triggered at that width and interferes with this style:

https://www.nafzinger.com/wp-content/plugins/responsive-twentyten/css/style.css
line 95:

#primary, #secondary
{
  overflow-x: hidden;
  overflow-y: hidden;
  width: 23.4043%;
}

Thanks for the quick reply. After digging some more, turns out I had installed the Responsive TwentyTen plugin ( https://gingerbreaddesign.co.uk/wordpress/responsive-twentyten/ ) some time ago. After disabling this, the problem resolved itself.