Other mistake: The view of Katherine's bloody body showed blood running diagonally almost exclusively from left to right, but the Ten Killer slashed her equally from right and left. Blood should have run in both directions. (00:45:05 - 00:46:25)
Other mistake: The "Ten Killer" slashed Katherine Hourback's face/head close to 50 times (off-screen), leaving her face covered in blood. When Dan found Katherine's body in his front yard, there was diluted blood on her face (mostly left side), but no indication that she had been slashed with a utility knife about 50 times. Also, during the slashing, the Ten Killer's "aim" seemed off (too high for a victim sitting in a chair and with no adjustment for how her head might move after each forceful blow). (00:45:05 - 00:46:53)
