Summary: Despite great progress in diabetes treatment, a significant number of patients still do not achieve optimal metabolic control. There are many factors and barriers. The aim of the article is to define and list errors related to diabetes treatment and self-care (examples from insulin therapy, nutrition and SMBG) often seen in clinical practice, and to describe possible causes and mechanisms. Some psychological aspects as possible problems are also described. The authors emphasize the importance of patient evaluation, focusing on mistakes which could be the reason the diabetes is uncontrolled. Thus, an „error checklist” form is proposed. The need for higher quality patient education and an improvement in patient-heath provider communication to reduce the risk of patient errors is also emphasized.