Yeah, this really did happen to me... And I'm not sure how I'm going to get it out. Does anybody have any good ideas how to do it? There is currently no beer in carboy. Just a damn bung.
In theory it can stay there. What you need to do is get a sharp knife, probably serrated, that will fit through the opening. Use a wire or something to pull it up, the stiffer the item the better (you might need two implements), and then work on cutting it down to a size that will come out. No miracle cures here, just elbow grease. And patience.
I thought about leaving it, but was concerned about any rubbery off flavors as a result. Honestly, I haven't spent too much time trying to pull out, didn't feel like it today after brewing. I think I might be able to grab with needle nose pliers. Also, this is a bung with no hole. Which makes it a little tougher.
Haaa! LOL, that does make it harder. Somehow, and I don't think I was involved in putting it in either time (maybe once), I had to do it twice. My other suggestion was going to be what others, including myself, have also done. Fill it with wort, and then move it to where you're going to ferment it. Gently, but with just enough pressure, set it down so that the bottom cracks and everything comes out - problem solved!!!
There is a neat trick requiring a large handkerchief, or a similar size piece of cloth. Get some cloth in the carboy and jiggle until the cork is enclosed, get it surrounded with the help of a coat hanger, gather the cloth and give it a good yank. If everything is a little damp it will pop right out. You will probably not succeed the first time but it will feel pretty cool when you get it out.
You could use a grocery plastic bag. Let it wrap around the bag by twisting the bag and once it's hung up pretty good, pull quickly! Try to line it up with the smaller end of the bung towards the exit. Good luck.
In case of emergency...break glass Seriously though, wire coat hanger and needle nose pliers should do the trick.
+1, a towel or kitchen towel works well too. I now use a large diameter blow-off tube directly into the mouth of the carboy (the other end in a pyrex full of sanitizer) for really active ferments, or if the krausen is less violent, a rubber carboy cover or a bung with a lip.