When Pullo pulls the man off the horse he immediately mounts the horse. But, when the scene changes to a long shot of Pullo finishing mounting the horse; the man he pulled off the horse is nowhere to be seen.
After the death of Caesar the slave Posca becomes a freedman and starts working for Mark Antony. However, under the Roman code of patronage freedmen remained in service with their former master and subsequent heir. So Posca should have started working for Octavian, not Mark Antony.
Marc Antony's face is reversed in a close-up shot when he is attacked at the Senate (a mole is on the other side of his face).