However, in all three instances,the skin lesions occurred under the cathode supraorbital regionat the end of the sessions. By separating the electrodes from the skin they presented small skin lesions, which resembled red burns, with small blisters Fig.1. The extension of the lesions ranged from 2 to 3 mm up to 1.5 cm. Lesions appeared after the second stimulation session in one patient, while for the other two, they appeared between the eighth and tenth sessions. None of the patients had a skin lesion before the start, skin disease or a history of any pathological skin disorder

via Skin Lesions Induced by Transcranial Direct Current Stimulation tDCS – 532.pdf.