Revealing mistake: When the detectives were listening to the message on the answering machine, we heard it say the date was Thursday and states a time the message was left. When the camera shows the answering machine, you can see the display blinking between 1 (the current message) and CL (which indicates the time and date have not yet been set).
Revealing mistake: When Karen is searching the net, she is on "www.tdn_japan.con" [sic]. This address wouldn't work... Also the Google bar spells "Imaegs" not "Images." (00:59:30)
Revealing mistake: In the scene Where Karen is taking a shower and fingers show up on her head, she jumps and on the bottom left you can see she is wearing something red, and is not naked.
Answer: Kayako was murdered by her husband when he found out that she has romantic feelings for another man, since this would be a crime of passion driven by hatred the cutting out her face from all the photos then sinisterly pinning them to the door is another sign of his hatred. Like him removing her from the family and cause futher pain with the needles. Also to display her face would most likely be a way of shaming her further, insult to injury and all that.