Most of the times, I encounter major translation issues when the original sentence is barely intelligible in the original language. Other times there's a small mistake which could be understood by context in the original language but it's completely lost once translated — like it happens when you try to translate jokes and puns or when you miss a comma.
People generally don't write correctly, so Google needs a lot of luck to understand that "Let's eat grandma!" doesn't actually translate to "Mangiamo la nonna"
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I translated this whole comment for the sake of it and the result is extremely good. Not completely natural but well beyond understandable.