It started as an honest mistake, became a diplomatic standoff, and brought down an Empire.
In 680, English bishops gathered at Hatfield sent Pope Agatho a signed copy the Creed in which they declared their belief that the Holy Spirit proceeds from the Father ‘and the Son’. They would have been horrified to learn that this little phrase was not in the original. Unfortunately, some at Rome had invested so much of their credibility in it that they were prepared to go to any lengths to save face — even if it meant bringing down the Empire.