Final answer:
The sources of error in Tim's case are error-likely situation and individual differences, stemming from work overload and possibly Tim's personal multitasking abilities and attention to detail.
Step-by-step explanation:
Based on the description provided, the sources of error in Tim's situation include error-likely situation and individual differences. The error-likely situation is created by the need to balance the demands of checking out customers with the task of completing paperwork, leading to a possible work overload.
Individual differences may include Tim's personal ability to manage multitasking, his attention to detail under pressure, or how well he has been trained to handle such tasks. It is not mentioned that the paperwork was inherently complex, so task complexity does not seem to be a source of error in this scenario.
Other issues like conflict and ambiguity in defining the managerial role, difficult work relationships, or environmental distractions at the electronics store could contribute to mistakes but are not explicitly mentioned in the description provided.