An unusual error that I happily stumbled upon was in the Italian translation of The Curious Incident of the Dog in the Night-time when I was a kid. At some point, an adult has had enough of the protagonist and exclaims "Bloody hell", or something like that. Which was translated with outright blasphemy in Italian, for some unknown reason. Maybe the translator wasn't having it, either. Successive editions, for obvious reasons, have toned the exclamation down.