The Translation of the Treaty of Waitangi
This was actually a story in The Tool Box Journal by Jost Zetzsche and is a longer version of a story from his book, Found in Translation.
The lieutenant-governor handed the parchment document, inscribed in beautifully intricate longhand, to the missionary and Bible translator Henry Williams as the sun set over the Northland region of New Zealand on February 4, 1840. The urgency was apparent: tomorrow morning 39 Māori chiefs would assemble to debate this document, the Treaty of Waitangi, that would decide the future of New Zealand, and it must be translated overnight. Literally.
All potential parties to this treaty were highly motivated to come to an agreement. The indigenous Māori population was in disarray, threatened by marauding white traders and whalers and ravaged by a series of bloody tribal wars, made all the more devastating and gruesome by the recent European introduction of muskets. In response to a plea to King William IV from the Māori chieftains, the British government had dispatched William Hobson to New Zealand in 1839 to establish a British colony. Now, after a marathon four-day session of crafting and re-crafting the English text, Captain Hobson was passing it off for a rush translation.
Williams and his son had a good command of the Māori language, having lived and worked in New Zealand for 17 years. But the language intricacies of diplomacy and governance would prove to present some nuances that Williams was ill-equipped to navigate.
Nonetheless, the next morning the Māori chieftains and British representatives gathered in a marquee on the lawn in front of the British Resident's house to listen to the treaty read first in English and then in Māori. At its conclusion the chiefs began their negotiations, with Williams explaining, persuading, and elaborating throughout the day and long into the night. As the new morning was about to dawn and their food had run out, all of the chiefs signed the treaty. During the next eight months government agents carried the treaty to other areas of the North Island of New Zealand, eventually gathering 500 more signatures of Māori leaders.
Unfortunately, the treaty failed on many fronts to provide what the Māoris had hoped for. In fact, their dissatisfaction eventually resulted in the formation of the Waitangi Tribunal, established in 1975 and still going on today, charged with making recommendations for reparations by the government of New Zealand.

After all the careful protocol and attempts at consensus, what caused this dissatisfaction?
There is more than one explanation, certainly, but one primary fault can be traced directly back to that hurried overnight translation. Consider this:
In the text of the original English treaty, the Māoris were to agree to
. . . cede to Her Majesty the Queen of England absolutely and without reservation all the rights and powers of Sovereignty. . .
which is translated as
. . . ka tuku rawa atu ki te Kuini o Ingarani ake tonu atu te Kawanatanga . . .
Even if you don't read Māori fluently, you might actually be able to spot a potential problem. You may be able to recognize that the last word of the Māori translation (Kawanatanga) is actually a transliteration - not of the English word "sovereignty," though, but of "governance" or "government" (wanna guess what "Kuini" and "Ingarani" mean?).
In a retranslation back into English, the text says:
. . . give absolutely to the Queen of England for ever the complete government . . .(translation by Professor Sir Hugh Kawharu)
The Māori leaders who had hoped for the installment of a legal system to protect them from the lawless behavior of foreigners and restore order into their own systems were ready to allow the British crown to take over the governance, but they were not willing to sign away their sovereignty of their land. Yet that is exactly what happened.
Why did this translation error happen? Some linguists argue that there were existing Māori terms to describe the concept of sovereignty (rangatiratanga or mana), but it's possible that Williams and his son were either not aware of those terms or were actually acting in the perceived interest of their own government back in Britain and therefore used the term they knew had a greater likelihood of meeting with Māori acceptance.
And the cost of all of this? 1 billion NZD in reparations. Read more here.