Electric fans rotate so as to push air forward, so putting a hand or face in the front would push it away without serious injury. The fan shown at the beginning has been rigged to run backwards.
On the outside of the door to Marcia's apartment, there appears to be a keyhole to a lock above the doorknob, but when the door is opened there is no other corresponding hardware on the inside of the door or the door's edge. This is a common shortcut set carpenters often made.
In the first hypnotism scene, the hypnotist tells someone that he cannot open his eyes. But through much of the time of the hypnotism, his eyes are open.
When Dodie is suspended in air, wires holding her body up can clearly be seen near Desmond's head. This is an unfortunate side effect of modern film restoration; things that were unseen in the original 35mm prints now become visible because of the increased sharpness.
The morning after Marcia goes out with Desmond, Dave stops by Phil's apartment and they discuss what Dave saw and heard in Marcia's apartment after Desmond left. But Phil was still in Dave's car outside Marcia's apartment when Dave went in. There was no indication that Phil was going home while Dave was in Marcia's apartment. So, there should have been no need for Dave to go to Phil's apartment. They should have had their discussion in Dave's car after he left Marcia's apartment.