If you suddenly cover an egg which has just been taken out of the refrigerator with hot water it will crack, so it is a good idea to warm up the egg a little first in warm water before putting in the hot water.
なお、冷蔵庫から取り出した卵に急に熱湯をかけると割れてしまうので、熱湯を入れる前にぬるま湯などである程度卵を暖めておくと良い。