What causes the SMS system to stop working and throw DB errors?
This error does show up in Civi 4.7.8 on Drupal 7.x.  @PeteRoss Thanks.  Haven't had chance to upgrade to 4.7.9 yet.

Had to send scheduled event SMS to members, so modified sql in if statement in CRM/Mailing/BAO/MailingJob.php

525  if ($mailing->sms_provider_id) {
  ....
  ....
  546                         OR       contact_a.do_not_sms = 0 )
- 547                         $aclWhere}";
+ 547                         $aclWhere";
  548     }

The scheduled SMS job completed successfully after the modification.