From Teflpedia

A mistranslation or translation error is an error, usually a lexical error that occurs in translation, usually due to L1 transfer.

This can be due to false friends, or it can be due to a mistaken literal translation, or maybe just incompetence.

To produce a translation error a student doesn't have to be translating a whole text, but translating a single word or phrase.

Mistranslations are quite common in subtitles.