Factual error: The whole Egyptian mythos and references are incredibly sketchy: at first it is said that Foyle was born "on the 21st day of Proyet." Leaving aside the fact that Proyet is a season rather than a month and that it simply would not translate automatically into a matching month and day of the Gregorian calendar, it still wouldn't be the 21st of December as stated because Proyet fell rather in a January to May range. Not just that, but in the second half of the episode it's simply identified as "Midsummer", when Proyet was part of Winter and obviously seasons in Australia are the polar opposites of seasons in Egypt.
Factual error: Ignoring the fact that there was no such Pharaoh as "Memses", in the cabinet inventory the missing item is listed as follows: "'Silver stirrup ring, in the reign of King Memses, Dynasty Five, 2600 BC." If it were 2600 BC, it would be Dynasty Three maybe, but not Five, which started just past 2500.
Plot hole: At the end of the previous episode Foyle infiltrated the Fisher household and cut the telephone line. However in this episode, taking place just the morning after, no mention at all of the event is made and Bert and Cec off camera phone the police with no anomalies reported.
Continuity mistake: When Miss Fisher is on her knees by the burial site of her sister, the ribbon in her possession is quite different from the one seen at the beginning of the series, which was worn out, with no hint of the shape and color of the blue one shown in this episode and the previous.
Continuity mistake: When Dot is holding Hugh at gunpoint, Hugo Johnstone-Burt is holding a pencil between his thumb and middle finger only when shot from the front with Dot's hands in the foreground. He never has it in the angle behind him, or wider shots. (00:34:40)
Plot hole: The coroner finds the stones "in the nasal cavity" after Miss Fisher's intuition, but he already had examined it in both cases due to the bleeding. Also, it's inexplicable how with the previous case being ruled as the death of a drug addict with no particular findings, the coroner would have still lying around the body to examine for autopsy even when Hugh said the body was found a week before.
Continuity mistake: When Miss Fisher gets face to face with Foyle for the first time, her arm goes from straight to slightly bent at the first cut after his mocking applause; in the reverse shot the arm is still straight and only then begins to curve. (00:39:30)
Factual error: Miss Fisher and the crew on 21 December 1928 dance to the glorious rendition of "I'm Sailing On A Sunbeam" by Des Tooley featuring Frank Coughlan, playing a record that was released only in 1930.
Revealing mistake: Jack lights up a match to see better in the darkness of his prison with Jane. As Jane gasps, the match is extinguished, but the additional light in the room remains the same. (00:41:50)
Continuity mistake: When Miss Fisher asks Dot to help her get out of jail, she is holding on to the bars in the close-up, but not in the wider angle. (00:34:10)
Answer: Louisa's watch was broken during the struggle to get away from her attacker.
Bishop73