Drupal: Error: Class 'SelectQueryExtender' not found in includes/pager.inc

If you find that you Drupal installation is WSOD-dead with the following error message:

then this means that you've messed up the Drupal (7, at the time of writing) registry somehow. You might have done this during an import/export operation where you forgot to export the registry and registry_file tables, or as I did, you have foolishly just emptied them to clear up issues with a missing class file that Drupal fastidiously continues to track.

Fixing this can be done by either reinstalling Drupal or by copying the contents of the two aforementioned tables from an ideally fresh installation of Drupal and loading them in the database of the afflicted installation. The latter did the job for me.

Good luck!