In the film, it is assumed DeSalvo was guilty, and it portrays him as suffering from multiple personality disorder and committing the murders whilst in a psychotic state. DeSalvo was never diagnosed with, or even suspected of, having that disorder.
It was known from the outset that Tony Curtis as Albert DeSalvo was the strangler, yet when one of the victims - 'Bertha Bloom' - talked to him through the intercom system before letting him into her apartment, as he pretended to be a plumber coming to look at her bathroom, the voice was clearly not Tony's.
It was somewhat surprising that the bite mark on DeSalvo's hand, sustained during his attack of one of his victims, was not compared to the teeth marks from that victim, who survived. This might have provided proof that he was the attacker.
The Coke Dana gives to Lyonel is flat. There is no hiss when she opens the bottle and no fizz in the bottle or glass when she pours it.
A shot of Tony Curtis, in a doorway, includes a 1966 Oldsmobile Toronado in the background, at a time (in the story) which is supposedly 1963-64 (shortly after JFK's funeral).
When Henry Fonda playing John S. Bottomly, during suspect filtering quoted *we don't want to throw the baby out with the bath*. This is a well known idiom which is actually *throwing out the baby with the bath water*. Relates back to a time where each family family member in turn would take a bath, the baby (surprisingly) the last. The meaning relates to something good being eliminated when trying to get rid of something bad. As a policemen Bottomly would not mis-quote this popular saying.