I could not believe that such a simple mistake was about to have such dire consequences.

我不能相信这样一个简单的错误将会造成这样可怕的后果。

youdao

That's when I realized that this was not a simple mistake but that, in fact, people had adopted this course of action consciously.

那时我才意识到那不是一个简单的错误，实际上，人们是自觉地接纳了这种行动方式。

youdao

This may include a simple mistake, a lapse in judgment or failure to exercise due diligence.

BBC: How human error can cause a plane crash

As far as Kaplan was concerned, Mearsheimer's embrace of Atzmon was a simple mistake.

CENTERFORSECURITYPOLICY: Mainstreaming anti-Semitism

But underlying all those tactics was a very simple management mistake News Corp. made.

FORBES: How Facebook Beat MySpace