Most mistranslations are accidental and due to an insufficient grasp of the languages involved. At the movies, I see this very regularly in subtitles, almost always when obscure slang terms are used in the dialogue. Normally it doesn't really matter, so no big deal but sometimes the translation totally misreads the intention of the sentence.
But when it is done on purpose, it is much more serious.
On the morning news in France today, there was coverage of President Trump's speech to Congress, and there was an excerpt about "America leading the world." The French translation that was given was about "America dominating the world" and that seems completely deliberate to me since the language is rich enough to express the nuances of such phrasings. As much as I do not like the American president, this mistranslation displeased me.
Playing my own devil's advocate, though, I can also imagine that the person who did this was just giving the correct translation of what Trump really meant.