Occasionally, a service technician will encounter a problem that may not be evident while he is at the jobsite. The problem may be intermittent, and it may not occur while he is on the job.
This can be quite frustrating for any technician. He shows up to solve a problem for a customer, only to find the system running normally with no signs of a problem. Then, several hours or maybe days later, the customer calls again complaining that the system still is not working.