不测之事（名词，正式）

(noun, FORMAL) An unexpected and undesirable event; unfortunate accident; mishap.

EXAMPLE: A typhoon or other natural disaster can be a terrible contretemps for locals and tourists alike on tropical islands with economies that depend heavily on tourism.

在严重依赖旅游的热带岛屿，台风或其它自然灾害对当地人和旅游者来说都是可怕的不测之事。

side effect

副作用

Something unwanted that happens secondarily as the result of something else that is desired.

EXAMPLE: Unfortunately, a bad headache can be a side effect of the drug I must take for my illness.

真不幸，严重的头痛是我为治病不得不吃的药物的副作用。