"What happens in rare cases" is "가뭄에 콩 나듯이 드물다"(ka-mu-me kong na-dŭt) in Korean.
When literal translation is, the beans grow in drought. As beans grows to shine more. Like the happening is rare to happen, the situation that is really rare to happen use this phase, It is compound word of 가뭄 (Gamum, drought), 콩 (Kong, beans), 나다 (nada, to grow), ~ 듯이 (as).