It seems so clichéd to describe poor subtitling as "lost in translation", but unfortunately that is what often happens. Since colloquialism and common expressions are frequently used in films, failure to understand these can result in a total change of meaning in the dialogue, for example:

- Why do you spy on me?
- I?d like to think that I?m watching your back.

Has become:
- Why do you always stare at me?
- I just love to gaze at your back.

Wow! A thriller has become a romance.

Or in another case:
It?s a matter of trial and error

Was translated as:
It?s a matter of lawsuits and mistakes

Sometimes because the subtitler has misunderstood one line, the context of subsequent lines - or even a large chunk of the film - is changed in order to make it flow. So the poor audience ends up watching another film with the script written by the subtitler.

Poor comprehension skills on the part of subtitlers can also affect the quality of teaching, when video is used as a teaching material. This line appeared in a foreign language subtitle of a culinary school video:
"This chocolate ganache is very versatile, you can use it to? or make the filling of a dessert called Ramekin"

In fact, the demonstrating chef says:
"This chocolate ganache is very versatile, you can use it to? or fill a ramekin"

Needless to say, mistranslating technical information could have serious consequences but? why should you care?