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.
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.
Narrator: Barry - had now arrived at the pitch of prosperity and by his own energy had raised himself to a higher sphere of society. Having procured his majesty's gracious permission, to add the name of his lovely Lady, to his own. Henceforth, Redmond Barry assumed the style and title of: Barry Lyndon.