We were getting errors in our drupal commons site on cron run. I tracked it down to heartbeat module - specifically line 467, and then the query that immediately follows. Errors had to do with the db_placeholders() function not having correct argument. The array that is passed must have at least one value.
I implemented the following change:
$unlimited_templates = variable_get('heartbeat_activity_templates_unlimited', array());
//my added fix - db_placeholders() requires an array with at least 1 item
$unlimited_templates = array(1);
Cron now runs without errors.. but honestly I didn't track this back very carefully. Hoping I didn't break something else by doing this?