There's a 2-block-high 4-block-wide space under monorails, which is plenty big enough for players and mobs to exist in, but it's impossible to get in or out of it without breaking a block somewhere.

I note that solution 2 to issue #77 will resolve this, as the gap under there would then only be 1 block high.

Fixed by 73004ad

I think mobs don't spawn on glass normally?