When a cube is defined in a "bad" way the front faces will be culled.

{
"__comment": "Fair warning, this format is highly likely to change even more in the future!",
"name": "cactus",
"useAmbientOcclusion": false,
"elements": [

{ "type": "cube", "from": [ 0, 0, 0 ], "to": [ 8, 8, 8 ], "shade": [ 0.5, 1.0, 0.8, 0.8, 0.6, 0.6 ], "uv": [ [ 0, 0, 16, 16, "down" ], [ 0, 0, 16, 16, "down" ], [ 0, 0, 16, 16, "down" ], [ 0, 0, 16, 16, "down" ], [ 0, 0, 16, 16, "down" ], [ 0, 0, 16, 16, "down" ] ], "cull": [ false, false, false, false, false, false ] }

,

{ "type": "cube", "from": [ 16, 16, 16 ], "to": [ 8, 8, 8 ], "shade": [ 0.5, 1.0, 0.8, 0.8, 0.6, 0.6 ], "uv": [ [ 0, 0, 16, 16, "down" ], [ 0, 0, 16, 16, "down" ], [ 0, 0, 16, 16, "down" ], [ 0, 0, 16, 16, "down" ], [ 0, 0, 16, 16, "down" ], [ 0, 0, 16, 16, "down" ] ], "cull": [ false, false, false, false, false, false ] }

]
}