What's wrong with this egg??

[ErinG]

I have an overabundance of eggs right now, so I decided to hard boil some of the older ones. I noticed one egg that I didn't recognize - I only have 5 girls and I can tell their eggs apart. My parents have my chickens right now, so I didn't collect the egg from the nest. Anyway, it was a little bigger than a banty egg and round. When I filled the pan with water it floated. I took it out and cracked it open out of curiosity and was surprised to see a deformed yolk. It doesn't smell rotten. I looked up fart eggs, and couldn't find anything on unusual yolks, just that they seem to not have them frequently.
Thoughts on what caused this? It seems to have been a one time fluke, but I'm interested as to possible causes.

 

[ChickensAreSweet]

If that egg floated, it is rotten.

http://avianaquamiser.com/posts/How_to_determine_the_age_of_an_egg/

Here is a nice site for fun:
http://edis.ifas.ufl.edu/ps020

 

[ErinG]

That is a fun site! The weird thing about that egg is that it had no foul odor.