Visible wires on floating casket.
Shortly after the corpse is stolen, a cane is moved over the corpse, and you can very clearly see eyelids twitch on the actor who is playing dead. It is so obvious, that it made at one viewer think the "corpse" is alive, or coming back to life.
Upon stealing Conrad Radzoff's body, the corpse is very limber and slumps as it is carried. A recently embalmed body would be stiff due to the hardening of embalming fluid.
During a scene towards the end of the movie where Conrad is walking up a staircase towards Stu, a hose can be seen spraying fog on the stairs.