When Derek/Ian ties up Brooke next to Peyton she is able to get the handkerchief off of her mouth. When Brooke says, "Untie me, you freak." you can clearly see the handkerchief around her neck. When it flashes back to Derek/Ian and he says, "Figure's you do good with your mouth." you can see Brooke and the handkerchief is around her chin.
When Peyton is tied up in her basement by psycho-Derek the night of prom, she tries to call Lucas with her cell phone, but Derek hear the phone ring and take it away from her. When the camera is on the phone, you can see the date on her phone who is 2/1/2007, several months before prom.
Camera seen lower right corner around 36 minute mark.
When Brooke is looking for Peyton inside the house, she believes Peyton is hiding from her and uses the proof that Peyton's curling iron was still on. However, Peyton was walking down the stairs when the doorbell rang, implying she was done getting ready, so her curling iron should have been off.