A dead chicken has a chance of still laying an egg.

I had a bunch of chickens around and they had all laid eggs.
I killed each and everyone of them. The last one I hit with an iron sword and after it died and fell over on its side, right as it disappeared, I heard a the egg laying sound and an egg appeared even after it had alright it disappeared

I'm sure that this will be hard to reproduce as you can't be sure when exactly a chicken will lay an egg.