Factual error: While Stanley Kubrick boasted about the authenticity of the candlelit interiors, he must have missed the little drawing of a steam train on a period map - 40 years before trains were invented.
Character mistake: When Barry Lyndon is talking to his mother privately, while standing on the bridge overlooking the water, Barry's mother refers to Barry's son Brian as being "at the mercy of his stepbrother", meaning Lord Bullingdon. They have the same mother. Lord Bullingdon and Brian would be half brothers, not step brothers.
Continuity mistake: Barry is introduced to Gustavas Adolphis, the 13th Earl of Wendover, by Lord Hallam, to help him obtain a title. In the receiving line to meet Your Majesty, another man is introduced as Lord Neville, yet later when Barry is eating in a restaurant and runs into the 13th Earl of Wendover, he calls him Lord Neville. It seems his name should be either Lord Wendover, as he was called previously, or Lord Adolphis, as we learned that his real name was from Lord Hallam.
Continuity mistake: When Lord Bullingdon and Barry Jr. are left behind by the chaplain while at their lessons, Lord B. is copying a text. The various shots showing his writing seem to be in the wrong order: in some shots there is a line of writing less than in the previous shot.
Revealing mistake: When Barry stands at his uncle's dinner table to supposedly "toast" Captain John Quinn for his pending marriage, to Barry's cousin Nora Brady (who Barry is in love with), he throws his glass in Captain Quinn's face instead. However, the glass noticeably breaks before Barry even throws it.