a mistake
This sentence contains an error.
O professor nunca deve ridicularizar um aluno que comete um erro.
A teacher should never ridicule a student who makes a mistake.
Isso talvez tenha sido um erro.
That may have been a mistake.
Assim que Tom pediu Mary em casamento, ele percebeu que aquilo era um erro.
As soon as Tom proposed to Mary, he realized it was a mistake.