First of all, there are at least five different YouTube videos where people try it out and have no problems. Here is one that includes various sizes and shapes.
Secondly, if this were possible, it is likely that cases would appear in the medical literature, but they don't. It is not difficult to find documented cases of light bulbs stuck in people's rectums, of children swallowing small LEDs and so on. Here is a dataset of nearly 2000 light-bulb related mouth injuries presented at hospitals. While only some descriptions are available and all of them are brief, they are just burns, cuts and the like. None of the available descriptions mention one getting "stuck".
EDIT: To be clear, cuts from light bulbs in the mouth don't necesserily imply that the bulb was stuck. Here are typical descriptions of the injuries.
- 2 [year old male] BIT A LIGHT BULB AT HOME. PATENTS GOT SOME PIECES FROM
MOUTH. DX-ABRASIONS MOUTH.
- 19 [month old male] BIT LIGHT BULB IT BROKE, HIT MOUTH. DX MOUTH CONTUSION
- 2 [year old male] PLAYING W/ A LIGHT BULB WHEN HE PLACED IT IN MOUTH, LIGHT BULB BROKE. GLASS CUT PTS LIPS. DX MOUTH LAC