This article provides an assessment of the problem of diagnostic errors in autism spectrum disorder. The fact that awareness of autism is on the increase has led to the emergence of a growing number of specialists, as well as other people professionally involved in education and the care of children, who have been noticing features of autistic disorders in ever-younger children. On the one hand, this is certainly beneficial in that the level of knowledge about the symptoms of autism spectrum disorder (ASD) is conducive to the identification of children’s difficulties; on the other, however, there are concerns that the knowledge possessed by many of those who come into contact with children is often insufficient to diagnose this subtle and changing spectrum of disorders. Neurodevelopmental disorders are especially difficult to assess in small children due to the overlapping symptoms of various disorders. Additionally, periods of intensive development or regression also overlap. Children aged two or three are still too young to exhibit some of the behaviors specific to ASD, and the assessment of speech development and its understanding may be deceptive. The diagnosis of “under observation for possible autism” is often exaggerated. The article presents case studies concerning diagnostic errors in autism spectrum disorder, their consequences and vital conclusions.