The best theory I can come up with is that the skin servers were down, or partially down, over the past few days, and the mob heads got caught up in this. The ones that worked would've been first rendered, even if you didn't see it, when the skin servers were up, and the rest when they were down.
I can confirm this too, as yesterday on FTB both my skin and KingNurtul's was not showing up to me, and Catty's was, but to him it looked like mine and his were fine.
If this persists, tell me and I'll see what I can do. Otherwise, all should be resolved as Jan said.
I highly doubt april fools affected it, but what im going with is everyone's skins changed. I am not sure if the heads changed but if they did, there is a 1000 to 1 chance it could of. I am just putting all the options I can think of together.
I doubt they would change everyone's skin. The was I assume they did it, or the way I would have done it, is to change the return value of the skin download call, so when the client requests minecraft.net/skins/luisc99, instead of returning 1acdc0ce-f391-4835-9214-0fad8064fe70.png, it'd return a random villager skin.
Yes, that's my player UUID. I used it because I mostly remember it due to some snapshot tests I've been doing.
