The job/queue only performs BULK operations. If a bulk action for a particular order is added to the queue, and a singular action is performed on the same order, the order in which these actions are performed depends on timing; either action could occur first.
If an error is encountered when starting the action, the scheduled job will continue through the batch of 200 actions, but will record the error in the job history. Upon completion of the 200 actions, the scheduled job will fail. This type of error is most commonly seen when another order is in process on the account.