We present a case of a preoperatively unrecognized giant bladder diverticulum in a 63-year-old postmenopausal woman. We carried out preoperative ultrasonographic evaluation and suspected an ovarian cyst. We performed two transvaginal ultrasonography scans before surgery, asking the patient to totally empty her bladder. Consequently, we decided to subject the patient to laparoscopic cyst removal. However, during surgery, the presumed ovarian cyst was not found and a bladder diverticulum was suspected. To confirm our hypothesis, we filled the bladder with saline until the reappearance of giant swelling and we recognized a bladder diverticulum. This case shows that diagnosis based on imaging alone can reveal unpleasant pitfalls. It is important to integrate clinical information, and to consider other possible investigations in order to reduce the chances of a misdiagnosis.