I'm playing Infinity Evolved Skyblock. While I was farming up my vanilla crops to 10/10/10 Agricraft versions, I found that my Mushroom/Netherwart enclosure was prime for spawning mobs. Not many, but I could go off for a walk around my perimeter and come back to find a few bats, maybe a hostile or three. I learned that four rotten flesh and a bit of smooth sandstone could be melted in the smeltery for 100 mB of blood to make Netherrack. Nice, but throwing a live Zombie in there probably gets you more (especially since you need at least two zombies to get 4 rotten flesh). Plus, it would get you something from Creepers and Skellies as well.
So, I set up a second smeltery for this, and slap two enclosures identical to the one that was spawning mobs on top of it. Walls are Dark Glass (so I can see if anything's in there) with a divider to keep the 'light' from the smeltery from getting in, floor is dirt - and sticky pistons are at the ready to yank the floor out. NEI shows me red X's, it's the same biome, only about 3 blocks higher up.
So, I set up a second smeltery for this, and slap two enclosures identical to the one that was spawning mobs on top of it. Walls are Dark Glass (so I can see if anything's in there) with a divider to keep the 'light' from the smeltery from getting in, floor is dirt - and sticky pistons are at the ready to yank the floor out. NEI shows me red X's, it's the same biome, only about 3 blocks higher up.

No spawns. Not even a bat. I've idled for an hour or more in the opposite corner of my base, which is even further away than I used to go when the farm was unintentionally producing mobs. I've even checked to see if my game spontaneously switched to Peaceful (nope, still on Normal). I'm flummoxed.