我不小心拿错了别人的书。

I picked up someone else's book ___by ___mistake.

我不小心付了两次账单。

I've paid this bill twice ___by ___mistake.

30分钟后，一个男人出现，告诉我们他不小心拿错了我们的行李箱。

After 30 minutes, a man appeared telling us that he had taken our suitcases ___by ___mistake.