What a huge egg! Do you know what kind of hen laid it? We have a BR that's laid huge eggs like this twice...the first one was a double yolk...the second one, I don't know because it's still in my fridge (I haven't finished showing it off yet...lol...)
I got one double the size of the eggs I've normally been getting lately just this morning. I think it's a "first egg" from one of my eleven hens that I've been waiting for to start laying. Congrats on that monster cackle berry
That egg is most likely either going to be a double (or triple) yolker or it could even be an egg within an egg. Very rarely, as the egg receives its shell, which is the last thing to happen, it will reverse enough that as the next egg is receiving its shell it is formed around the last egg. When this does happen the egg is huge.
Make sure to post a pic of what it turns out to be.