I was fooling around with gamerules, when I noticed this:
I guess I added a gamerule to the list? I'd like to know more about this and how it can be used.
Custom Gamerules makes you add other rules. This is an example:
Enable/Disable every zombie being a baby
/gamerule onlyBabyZombies 1/0
That is one of the custom gamerules. Tutorial on how to make a custom gamerule: http://www.minecraftforum.net/forums/minecraft-discussion/redstone-discussion-and/2353772-tutorial-make-your-own-custom-gamerules