Factual error: At the end of the movie, Lady has four puppies, three Spaniels, like her, and one mixed breed, like Tramp. Realistically, they would all be mixed breed with characteristics from both parents.
Factual error: When Kitchener is shown inspecting Harry's old regiment, one group of African or Egyptian troops is shown to be wearing the Pattern 1914 Infantry Equipment, not made until 1914 at the earliest. Additionally, it has the 1918 modification to have two closing straps, so is over 20 years too late for the 1894 date at that point.