Module: mm_s3
Function: mm_s3_delete_file
Line: 341
The foreach($delete_files as $bucket => $file) is not correct. There's an additional array in there. I also added an error check so this does not happen invisibly. You may wish to implement the success differently, the foreach statement is the important part :)

  $deleteSuccess = TRUE;
  foreach($delete_files as $file) {
    $deleteSuccess = ($deleteSuccess and $s3->rmFile($file['bucket'], $file['file']));
  }

  // if we successfullly deleted the files
  if($deleteSuccess) {
    drupal_set_message(t('Deleted S3 files for !mmfid, node !nid', array('!mmfid' => $item['mmfid'], '!nid' => $item['nid'])));
  } else {
    drupal_set_message(t('There was an error deleting S3 files for !mmfid, node !nid', array('!mmfid' => $item['mmfid'], '!nid' => $item['nid'])), 'error');
  }