Continuity mistake: After the doorbell knob retracts into the frame, the door starts to open. After the angle change, the door starts to open again.
Other mistake: As Fester goes to shoot Gomez, his aim strays upward to about a 45 to 50° angle. Not enough to shoot the support chain on the light that is directly above him, and hits him on the head.
Continuity mistake: As Fester gets a pistol from the drawer, there is an assortment of items in the drawer, and he leaves it open. A moment later, he goes to exchange guns, the drawer is closed and suddenly bare, except for the other pistol.
Continuity mistake: As Fester talks with Gomez, he has a fork in hand that has a small chunk of meat on it. In one quick take, there is an entire meatball on it, then a moment later, it's a small bite again.
Continuity mistake: As Fester talks with Gomez, he puts the fork with hamburger on it down in the bowl he's holding, and his right hand under the bowl. A second later, he's suddenly holding the fork.
Answer: Basically from what I understand, they stuck the innards of a small flashlight into the base of the bulb and put a little switch on it. When he put it in his mouth and bit down, his teeth would hit the switch, which would turn on the flashlight and made the bulb light up.
TedStixon