How often does ovarian cyst get mistaken for ectopic pregnancy?

Shouldn't be often: I don't know of clinical studies that addressed this - maybe another doctor here does - my educated guess is that it shouldn't be often. Remember most ectopics are diagnosed by combination of abnormal quants and ultrasound, so you'd need abnormal quants plus a cyst to mis-diagnose. The reverse, mistaking an ectopic for a cyst, would be more plausible.

Never by expert: Each has its own distinct features by sonography , trained expert will not get mistaken , besides location.