The engagement ring Murdoch looks at toward the end of the episode has not only a different stone, but a different shape. At the end of season three, the stone was a round sapphire. In this episode, it's a ruby marquee.
When the homeless guy, Arthur Frumm, is being questioned, he lies down on the table. One of the rims on his glasses is obviously broken. When he gets up, the rim is repaired.