The original Swedish translation by Åke Ohlmarks is known world-wide for its huge errors. These errors include Legolas having children, Elrond's wife being dead and Merry killing the Witch King instead of Éowyn. The books were re-translated a few years back but the new versions were heavily criticized for re-translating some of the names (Baggins to Säcker rather than the previous Bagger, Brandybuck to Brännbock instead of the earlier Vinbock, among others) and many of the fans prefer the original translation, even with its heavy errors. See more...
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The Lord of the Rings - 0 mistakes
I'm afraid there aren't any mistakes listed for this book.