Background: Tattooist-related technical failures of tattooing were hitherto unstudied and related to clinical complications. Tattooing requires talent, training and experience. Amateurism is a challenge in popular tattoo industry with no formal education and certification of the tattooists. Objective: To study technical tattoo failures causing disease in a consecutive hospital material of tattoo complications. Material: 574 patients with 702 tattoo complications referred to the “Tattoo Clinic” (a subspecialised dermatological clinic) were enrolled. Patients were examined clinically and classified with respect to the cause of complication. Results: 147 (21%) tattooist and tattoo studio-related complications were recorded, i.e. excessive pigment installed in the dermis with “pigment overload” in 64 (9%), tattoo “needle trauma” with “overworked tattoos” in 43 (6%), contaminated ink causing infection in 20 (3%), and other sources of infections related to tattooing in 20 (3%). Pain and discomfort were particularly common as well as long-term complications including scarring induced by “needle trauma.” “Pigment overload” with black pigment carried a special risk of granulomatous inflammation and sarcoid granuloma and was observed in 12/35 (34%) of punch biopsies taken from tattoos with “pigment overload.” Keratoacanthoma associated with trauma was observed in 1 case. 82% of complications were related to professional tattooists working in a tattoo studio and 18% to amateurs. Conclusion: Technical failures of tattooing are associated with medical tattoo complications. “Needle trauma” with major skin damage, e.g. “overworked tattoo,” and installation of excessive pigment, e.g. “pigment overload,” and (re)use of contaminated tattoo ink bottles are identified failures calling for preventive intervention.