William Pitt's deathbed scene: when Pitt says "I'm scared", a blob of pale yellow discharge can be seen beneath his left eye. The shot then cuts to Wilberforce, but as soon as it cuts back to Pitt (after about a second) we can see that the blob has vanished.
Charles James Fox was never "Lord Charles Fox". He was in the House of Commons until he died.
The pro-slavery Clarence was a royal duke (i.e. one of the king's younger sons) and thus could not have been a member of the House of Commons.
The movie originally portrays Banastre Tarleton, the Liverpool MP, participating in a Commons debate in 1782. Tarleton did not enter the House of Commons until 1784, and could not have debated on negotiations with Americans as he was not yet an MP and was in fact, on parole from his disastrous performance in Virginia.
As a younger son of the monarch, the Duke of Clarence would have been addressed as Your Royal Highness; he is referred to as Your Grace early in the film.
Charles James Fox died 13th September 1806 and so could never
have made his comments about William Wilberforce after the abolition of the slave trade.
At several points in the movie we see handshakes, notably Wilberforce with other MPs. This is entirely in period; there are references to shaking hands at least as far back as Shakespeare. It was unusual in other European countries (the young Wilberforce and Pitt amused the French by doing this when visiting the court of Louis XVI in 1783) but not in Britain.
Near the end of the movie, Clarkson is sitting at the grave site of Equiano. While he's talking, the blowing wind causes the cardboard or foam headstone to move.
Wilberforce's reference to "millions of years" doesn't fit. Even non-Christians of that period would not have thought in those terms.
During the boat trip to see the slave ship, servants are shown pouring from the distinctive bottle of Dom Perignon champagne. However, the brand was not introduced until 1936.
Towards the beginning of the film, Barbara Spooner refers to Napoleon as the ruler of France. This scene takes place in 1797, two years before Napoleon came to power.
A Member of Parliament says "bloody" on the floor of the House of Commons. At the time, the word was quite obscene; this never would have happened, and if it had, it would have resulted in the disgrace of the member.
The tune to which the song Amazing Grace was sung was not used until some fifty years later.
During the hymn Amazing Grace sung at the wedding, Barbara is seen singing "but now am free" but is heard singing "but now I see".
A character mentions seeing Germany with a telescope, however no unified German state existed before 1871.
There were at least 3 grammar mistakes: several uses of "I" when "me" should have been used. Wilburforce also says "By who?" As a graduate of Cambridge, he would have said, "By whom?"