If you thought Snakes on a Plane was terrifying...
But in all seriousness, when spawning mobs, make sure to check to make sure both the bottom and the top block are clear. If it's not that, then I don't quite know what did it. :|
I think its a glitch with the spawning code. I've had a skeleton stuck in a glass floor i loaded from a sandbox game and im 99.999999999999% sure that didn't grow while I had my back turned. Stupid skeleton took out 5 hearts too...
But in all seriousness, when spawning mobs, make sure to check to make sure both the bottom and the top block are clear. If it's not that, then I don't quite know what did it. :|
There are no mobs in survival