When my Queenie starts up laying or has a burp in her system (hey, burps and farts in the same thread!) she lays eggs that aren't quite fully colored. I think fart eggs are kinda the same, they tend to occur when there's a "burp" in the system!
Every once in a while I will get one and I give to my customers who have small children. I tell them the hens laid an egg just for them. Kids love fart eggs.