Commonest complication of Zenker's diverticulum is: (A) Dysphonia, (B) Gastroesophageal reflux, (C) Lung abscess, (D) Perforation
Answer is C. "Pulmonary infection is the most frequent serious complication, and many patients are first seen after experiencing repeated episodes of pneumonia."-CSDT

Aspiration pneumonia & Lung abscess are the most frequent complications of Zenker's diverticulum.
Other rare complications are:
Perforation
Bleeding
Carcinoma