What other common or serious conditions mimic appendicitis?

Several. Crohn's small bowel disease illeal lymphadenitis meckel's diverticulitis tubal disease in females diverticulitis pelvic inflammatory disease gonorrhea kidney stone on right many other items.
Ruptured cyst/Crohns. It is often difficult to distinguish inflammation in the right ovary or fallopian tube from appendicitis. Therefore, a ruptured ovarian cyst or pelvic inflammatory disease can look just like appendicitis. In men or women, inflammatory bowel disease or diverticulitis can have similar symptoms and signs. Though not perfect, cat scans have helped differentiate these diseases.