I can reproduce this with multiple fields, but I have not tested on other modules. For the tasks module if you add a custom field then create a workflow that has a condition based on that custom field you will get an SQL error like this:
SELECT tasks.id AS id FROM tasks WHERE tasks.name = ‘Verify Eligibilty’ AND tasks_cstm.success_c = ‘1’ AND tasks.status = ‘Completed’ AND tasks.date_modified > ‘2014-04-23 20:47:03’ AND tasks.date_entered <> tasks.date_modified AND NOT EXISTS (SELECT * FROM aow_processed WHERE aow_processed.aow_workflow_id=‘647fd903-0fbd-09dc-43d0-535826f4e50c’ AND aow_processed.parent_id=tasks.id AND aow_processed.status = ‘Complete’ AND aow_processed.deleted = 0) AND tasks.deleted = 0 : MySQL error 1054: Unknown column ‘tasks_cstm.success_c’ in ‘where clause’
It just seems that the workflow logic should join custom fields and it would be taken care of. TIA