Dornwinkle (the Invisible Maniac) tosses a radio into the water on the shower room floor, electrocuting one of the girls as she's taking a shower. A few scenes later, the same radio (the masking tape on the speaker gives it away) is on a table in the cafeteria, playing tunes as two students eat lunch.
Chet has a fight with the invisible maniac and is able to him once on the nose (Ouch, my nose!). When the maniac becomes visible again, he has blood coming from the corner of his mouth, but no blood is coming from his nostrils.
The professor's clothes disappear after he injects himself with the serum. However, his blood becomes visible when he is punched and spits some onto floor.