Uncaught Error: Call to undefined function cache_get()

Note to self and others encountering a "PHP Fatal error" with Drush, e.g. something like this:

PHP Fatal error:  Uncaught Error: Call to undefined function cache_get() in /var/www/{mysite}/includes/module.inc:763
Stack trace: 
#0 /var/www/{mysite}/includes/module.inc(963): module_implements('system_theme_in...')
#1 /var/www/{mysite}/modules/system/system.module(2531): module_invoke_all('system_theme_in...')
#2 /var/www/{mysite}/includes/theme.inc(798): _system_rebuild_theme_data()
#3 /var/www/{mysite}/includes/theme.maintenance.inc(57): list_themes()
#4 /var/www/{mysite}/includes/bootstrap.inc(2965): _drupal_maintenance_theme()
#5 /var/www/{mysite}/includes/errors.inc(177): drupal_maintenance_theme()
#6 /var/www/{mysite}/includes/bootstrap.inc(2690): _drupal_log_error(Array, true)
#7 [internal function]: _drupal_exception_handler(Object(Error))
#8 {main}
  thrown in /var/www/{mysite}/includes/module.inc on line 763
Drush command terminated abnormally due to an unrecoverable error. 

In some cases there is a simple solution: Drush needs to be called as root or with sudo.