Other mistake: On the first morning at Manderlay, as the girl is leaving the dining room, she slips and nearly falls but Frith is there to catch her, but Frith was way into the dining room; there is no way he could have been close enough to catch her.
Other mistake: Noah, the brother, just disappears from the movie with no explanation at all.
Other mistake: It's the Professor's 60th birthday and the candles on his cake must be the trick type - they keep going out as the cake is carried into the room, but they quickly come back on again. But when it comes time for the Professor to blow them out, there's no problem - he blows them out easily.