Dressed to Kill (1980)

Plot hole: After his mother's death, Peter sets a camera up outside her therapist's office, so he can see who goes in and who comes out; he thinks his mother's killer might be one of the doctor's patients - a tall woman with long blonde hair. All he sees is people going in and people coming out, and then he sees her - the tall blonde. Problem: He never saw her going in, only going out, which is critical, because the killer is the therapist - a tall man - wearing a long blonde wig and make-up. Of course no-one says anything about "We never saw her going in, only out, so it must be the therapist!"

kh1616

Doctor Robert Elliott: Doctor, I am not paranoid. Bobbi was making threats over the phone. She said she's going to hurt me. My patient was slashed to death. And now my razor is gone. Now you don't have to be a detective to figure it out, do you?