At the boat race Mike Johnson Jesse Birdsall is wearing the crown of his king Neptune's costume, but in a further scene he is just putting the crown on.
The Medical Examiner says the teen-aged girl was stabbed, puncturing the heart, killing her instantly. But the scene of her murder clearly shows her being stabbed in the abdomen. Unless she is deformed, her heart isn't down there.
When DCI Barnaby inspects Christa's room, he goes straight to it without being shown, or given directions. But he would not know which room was Christa's. He has never been there before.
When Bob Moss surprises DCI Barnaby at the location of Richard's murder, Barnaby reminds him that it's still a crime scene. Bob says, "I'm used to making sure all's well before it gets dark." But it's already quite dark.