When trying to run "/execute @e[type=Chicken] ~ ~ ~ fill ~-1 ~-1 ~-1 ~1~1~1 minecraft:glass" (this is the example command from the snapshot post on mojang.com), it will work for all the chickens I find within loaded chunks, but I also get several "Cannot place blocks outside of the world" errors. Again this is in addition to several successful "27 blocks filled" and so on, and all the chickens (or whatever mob I am targeting, for that matter) I find are encased in glass. As a note, I was in singleplayer, creative mode, and on a world generated in 14w07a.