Factual error: The filibuster scene gives the impression that the Senators were powerless to stop Jefferson Smith's speech. However, they can make a speaker shut up if at least 3/5 of the Senators vote to. From the Senators' reactions, a lot of them probably would have voted to silence Smith. (Clearly, if this had happened, the movie would be much less interesting).
Revealing mistake: When a car drives by a store front and two hand grenades are thrown at it, the hand grenades bounce back into the street before the store blows up.
Character mistake: On the proclamation announcing a plebiscite on a proposed "monarchy", the adjective "monarchical" is misspelled "monarchial".
Factual error: Towards the beginning of the film, when Kitchener is shown inspecting Harry's old regiment, one of the 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.Andrew Upton