Actually a controversal in my country: One of the newest chapter of Spy X Family, the line Anya said was along the lines of "I can read minds." and the translator made it as "Anya can read Damion's heart." and it got so much backlash lol. That's one example of "unforgivable" because the original line was meant to be taken seriously
For example, in one panel of Hell Mode chap 51, I translated "That's why we're here to defeat you!" to "That's exactly why we're jumping you!", a bit of a modern slang but it's funny and the original line was still see-able.
But most definitely, if the translator mislead the story because of a joke, he or she must be the 3rd criminal along Jesus that history didn't taught us about.