One of my favourite instances of adaptation got to be Ted Woolsey's "son of a submariner! They’ll pay for this...", for the English localisation of Final Fantasy III / VI.

In the game, Kefka (the villain) is saying this as the heroes escape him, but the original only says "ヒーーー　くっそー！このかりは必ず返しますよ!"; literally "heeee shit! I will definitively return this debt" or similar. However:

• That "ヒーーー" interjection has no meaning on its own. It's only there to highlight the character's emotional state. It could be safely removed, without loss of meaning.
• くそ / 糞 kuso "crap! shit!" is vulgar, but by no means as vulgar as English "shit". Specially given the 90s, and this game being marketed to kids. But it means the villain is being rude towards the heroes (makes sense, right).

So, translating it as simply "hey you!" or similar would mutilate the original, by removing the rudeness. But at the same time, Woolsey couldn't use "shit" or "crap" or similar. So he looked at the context:

• Kefka is crazy, and the way he uses Japanese in the original is odd. For example, he uses the pronoun "ぼくちん" bokuchin to refer to himself, as if he was a kid - and yet he's a court mage of an empire dammit. (It's a bit deeper than that, but let's focus.)
• a bit before Kefka says this, there's a city in the desert also fleeing Kefka - by going underground instead, as if it was some sort of "sand submarine".

So Woolsey went with "son of a submariner!", something he likely made up on the spot. And you know what? It's perfect - it's completely on-character for Kefka to insult people in such a weird way.