Nothing is quite so frustrating (and embarrassing) as losing your bung in your bunghole. Well it really shouldn’t happen. If the bung goes all the way in, it means you’re pushing too hard. Here’s what is happening:
The standard bung size for a glass carboy is #6.5. A #7 will also fit.
Many of us use an acid sanitizer like Star San. Basic sanitizers like One Step are also very effective, but they’re slippery. After you sanitize the bung (rubber stopper) with slippery One Step and place it atop the carboy, it has a tendency to keep popping out. The mistake is to push the bung halfway (or all the way) into the neck of the carboy. The solution? Just allow the bung to sit on top of the carboy until the sanitizer dries. As it does, the bung will set itself into the whole nicely without any force. If need be, you can go back and adjust the bung later on, once it’s dry.