Drupal: visibility of side bars and custom regions on 404

Drupal hides the left and right side bar on a 404 not found error. In itself this may be a good thing because when your site gets hit with a lot of 404's (not necessarily by humans) it can become a performance issue.

Drupal does this in includes/common.inc with drupal_not_found():

  // To conserve CPU and bandwidth, omit the blocks.
  print theme('page', $return, FALSE);

Any custom region that you create will not be hidden, just the left and right side bars. Depending on what you're after this may look a bit odd.

This problem can be solved with the block visibility for the individual blocks that you added to your custom region. If you set up your custom 403 and 404 pages in a logical way (admin/settings/error-reporting) — e.g. error/403 and error/404 — you can simply change the visibility settings for the blocks (admin/build/block).

If you want full control over what blocks are displayed regardless of the region simply rename your left and right side bars to something else using the theme's .info file:

;----------------
; Theme regions
;----------------
regions[left]     = Left sidebar
regions[right]    = Right sidebar
regions[content]  = Content
regions[header]   = Header
regions[footer]   = Footer
regions[bottom]   = Bottom block bar

If you add a region, you need to define all regions, including the existing ones.

Rename the default left and right regions to something else and you can control the visibility of all the blocks in that region with the visibility setting.