If you're using Context module in your site, and Zen as your base theme, when you upgrade to Context 3, your site will probably broke, this can be fixed quickly by editing your template.php and add this code in your hook_blocks:

<?php
function <THEMENAME>_blocks($region, $show_blocks = NULL) {
    if (
module_exists("context")){
    
     
// Since Drupal 6 doesn't pass $show_blocks to theme_blocks, we manually call
      // theme('blocks', NULL, $show_blocks) so that this function can remember the
      // value on later calls.
     
static $render_sidebars = TRUE;
      if (!
is_null($show_blocks)) {
       
$render_sidebars = $show_blocks;
      }
    
     
// Bail if this region is disabled.
      //$disabled_regions = context_active_values('theme_regiontoggle');
      //if (!empty($disabled_regions) && in_array($region, $disabled_regions)) {
        //return '';
      //}

      // If zen_blocks was called with a NULL region, its likely we were just
      // setting the $render_sidebars static variable.
     
if ($region) {
       
$output = '';

       
$plugin = context_get_plugin('reaction', 'block');
  
       
// Add any content assigned to this region through drupal_set_content() calls.
       
$output .= $plugin->execute($region);
  
       
$elements['#children'] = $output;
       
$elements['#region'] = $region;
  
        return
$output ? theme('region', $elements) : '';
      }
    }
    else {
        return
zen_blocks($region, $show_blocks);
    }
}
?>

For more information take a look at this issue: http://drupal.org/node/633778