Idiopathic solitary cysts have a predilection for long bones and the mandible. Although the origin of these cysts is unclear, the prognosis seems to be good, given proper diagnosis and surgical treatment. A 14-year-old female patient with a bone lesion in the right mandibular ramus was referred to the Department of Pediatric Dentistry at the Medical University of Gdansk, Poland. Due to severe headache lasting two days, the patient had previously undergone magnetic resonance imaging (MRI) examination of the head. A unilocular bone cyst in the mandible was discovered incidentally. Comparison with a previous orthopantomogram (OPG) taken two years earlier for routine dental treatment revealed a clearly visible but smaller bone lesion at the same location. Surgery was carried out in order to achieve bone regeneration. This report documents the importance of careful evaluation and assessment of all radiographs taken, as the lesion could have been diagnosed and treated much earlier, thus lowering the risk of complications.