I am not sure if this has been reported, but some of the guys in my server found it, they could only replicate it with the mod and were not able to do so in vanilla game.
So the problem here is that when you dig a hole in the ground, make a tunnel around 1 block high and then crouch in there you can see through world. This is a mod/game breaking bug as you can also shoot zombies from there without any collision, loot any treasure or in PvP servers you can actually kill players.