In the surgical treatment of lung cancer, systemic mediastinal lymph node dissection, as one important routine procedure, has been accepted by most peers in the world. However, due to the special location of some mediastinal lymph nodes, the difficulty of dissection, and the negative preoperative CT results, the specific scope of lymph node dissection is still controversial. Especially the second group, which is located at the top of thorax, is likely to be overlooked for the above reasons. Here, we report a case of lung adenocarcinoma in which the preoperative CT showed no abnormal lymph nodes in the second group and also no enlarged lymph nodes of the second group were found during the surgery, yet lymphadenectomy was still performed according the routine with the lymph node of station 2 being sampled. In the postoperative pathological report, cancer cells were found in the second group, instead of lung adenocarcinoma, these cells come from thyroid and were proved to be papillary thyroid carcinoma, which is unusual because no obvious indication of thyroid carcinoma was found in preoperative color doppler ultrasound of superficial lymph nodes.