The Addams Family Meet the VIPs - S1-E11
Revealing mistake: As Morticia and Gomez throw the knives at the target, and they stack tip to hilt, we can see the wire they slide on.
The Addams Family Meet the VIPs - S1-E11
Revealing mistake: After Mr. Harris leaves the play room, Fester goes to lay down on the bed of nails. As he does, the squared outline of the board under his robe he lays on is visible.
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