When the maid accidentally walks into a small table when answering the door for Father Brown, two objects fall on the floor. As no further reference is made to them, it can only be assumed that it was not in the script, but for some reason it was not re-shot.
Although Nero reportedly twice tried to have his mother Agrppina drowned in a rigged boat ass "The Head of Caesar" suggests, these attempts failed, and she was murdered by one of Nero's assassins.