Missing "region" causes Undefined property: stdClass region in template_preprocess_block()

template_preprocess_block() in block.module includes this bit to count how many blocks are in a region:

  // All blocks get an independent counter for each region.
  if (!isset($block_counter[$variables['block']->region])) {
    $block_counter[$variables['block']->region] = 1;
  }

But when Context module renders blocks in context_reaction_block, it doesn't specify the "region" property on the $block object. This causes blocks when rendered to throw a PHP notice. This only occurs if the block.module is still enabled on the site (as Context module can work without it and avoid this problem).