An error, surly occurs when posting before Goggle'ing the question. A bug is what happens internally when reading a post like that. A defect is is. For software: I'd say an error, a bug, and a defect are equivalent in meaning. OK, a quick train of thought: IMHO For Web Apps and DB services, etc: An error may be due to server timeouts (load tests?) yet the App. software handles the error and sends the request again. If the error is not handled then the application has a bug and this bug could lead to unexpected results which get stored in your DB or sent to another Web Service, etc., which will cause unexpected results returned from those services. The bad data passed in and the results passed back are defective. Any work done using those results will lead to products built with the wrong data. These products are defects.
Do social engineers have quality assurance?