We present a case of non-obstructive bladder diverticulum in a 75-year-old post-menopausal woman. An ovarian cyst was previously suspected, which resulted in a futile exploratory laparotomy without making any definite diagnosis, 1 year earlier. During this admission, transvaginal ultrasound-guided cyst aspiration was arranged to determine the nature of the presumed 'recurrent' cyst and to relieve the symptoms. Prior to cyst aspiration, up to 700 ml of urine through urinary catheterisation and the gradual disappearance of the 'cyst' alerted us to the possibility of a bladder diverticulum, which was later confirmed by retrograde cystography. This case illustrates the lessons that despite considerable researches having been done on enhancing sonographic accuracy, diagnosis based on imaging alone is likely to be associated with multiple pitfalls. Recognising the common pitfalls and integrating clinical information and alertness with ultrasonic features remains the mainstay of sonographic differential diagnosis.