an unusual habit or part of someone's personality, or something that is strange and unexpected

You have to get used to other people's quirks and foibles. 你得习惯别人的怪癖和小缺点。

There is a quirk in the rules that allows you to invest money without paying tax. 这些规章的奇怪之处在于其允许人们投资而不必缴税。

By some strange quirk/By an odd quirk of fate (= unexpectedly), we ended up on the same train. 说来奇怪，我们结果上了同一辆火车。