Acute appendicitis is the commonest non-gynaecological surgical emergency in pregnancy. However, the concurrent occurrence of acute appendicitis with a heterotopic pregnancy is a rare event and presents diagnostic challenges to unsuspecting clinicians and sonographers. We present a case of a woman who had a heterotopic pregnancy and was noted to have a gangrenous appendicitis at laparotomy, illustrating how a diagnosis of acute appendicitis could easily be missed in pregnancy. We report the case of a 34-year-old woman in the first trimester of pregnancy who had a missed diagnosis of acute appendicitis after she had complained of vague abdominal symptoms for three weeks. She presented to a gynaecologist with vaginal bleeding for three days and was noted to have a heterotopic pregnancy on ultrasound scan. At laparotomy, she was noted to have a gangrenous appendicitis with pyoperitoneum concurrent with a ruptured left fimbrial ectopic pregnancy. Left salpingectomy and saline lavage were done and she had uneventful post-operative recovery. Unsuspecting clinicians, in patients without risk factors, can miss both heterotopic pregnancy and acute appendicitis. As assisted reproductive techniques become widespread, the possibility of heterotopic pregnancies must always be considered and any patient who presents with vague abdominal symptoms must be suspected to have the possibility of acute appendicitis. Because of the unreliability of laboratory investigations and clinical predictive scores in pregnancy, sonographers must be specifically asked to scan for heterotopic pregnancy and appendicitis in suspected cases.