One of the most disgusting discoveries I've ever made:
Why does Project Gutenberg's translation of The Three Musketeers need to translate écu as crown when the word actually means shield? (See also the cognates escutcheon in English and escudo in Spanish, all from the Latin scutum.) It doesn't translate livre to pound, does it? Yes, a numismatist will tell you that Louis XIII's French écu and Charles I's English crown were vaguely similar in value (3.23 vs. 2.06 grams of gold, according to those links, but GURPS Swashbucklers lists them as being around 20 $ vs. 25 $* over the entire 17th and 18th centuries) and that the écu displays on its reverse side not just a shield but also a crown atop that shield—but écu is not couronne, and shield is not crown.
Years ago, my parents bought an awesome Oxford collection of the Sherlock Holmes stories, in which every book included copious endnotes regarding definitions of archaic words, explanations of bygone cultural customs, and discussions of differences between the manuscripts that were used to compile the collection. Why don't more translators document their controversial decisions?