During the chase after he rescues the dauphin, Percy reacts as if he's shot, but his clothing shows no sign of damage from the bullet. Also, flintlock pistols had a very short effective range. If a soldier had really hit him, Percy couldn't have been so far ahead that the soldiers didn't see where he went after the wagon tipped over.
When the Scarlet Pimpernel is helping the family to escape in the coffins, the man gets into the coffin on the right. When the coffins are reopened, the man emerges from the coffin on the left. And if you watch really closely, you'll see the mother and son's coffin is reversed head to foot.
The Young Dauphin, Louis XVII, Louis-Charles de France, died on 8 June 1795 in the Temple prison after severe mistreatment.
During the ride to the picnic, tracks from mowing equipment can be seen in the neatly cut grass.
When Chauvelin returns the earring Marguerite lost in the library, you can clearly see she is still wearing two earrings.
When Sir Percy's courier is shot, he can be heard screaming. But when his face is shown, his mouth is closed as the scream continues.