When i was playing Multiplayer (which might of been the reason for this) i accedentally clipped through a cactus and i did it again after.
I thought it would just hurt me, but it also clipped me through.
for the 2x1 hitbox, i just noticed it, but when i tried to punch the invisible part of the hitbox nothing happened
1. enable flight or enter creative
2. fly down (maybe diagonally downwards) onto a cactus
3. you will be clipped through the cactus (but you don't get x-ray vision)
i am not 100% sure if the server is 1.8 or 1.8.3
**I have found this to happen while i was in 1.8.6**