When a cake is placed near the edge of the world, it's hitbox is doubled. You can also place another cake in the side of the hitbox that isn't occupied, without a problem like you normally would have placing a block inside another block's hitbox. Sometimes, a cake's hitbox will only render on one side, and a few pixels off. It's all very weird, I'm sure theres more, you will have to play around with it yourself. I know a similar issue was fixed recently, https://bugs.mojang.com/browse/MC-45346, this is different. Also, sometimes you can walk straight into the cake. Some of the hitbox issues seem to be directional.