First, I was trying to make a creeper farm, and though I had the carpets down to prevent spider spawns, a spider spawned. This may have been caused by an issue with trapdoors as well.

Then, I was in a slime chunk, where I have torches on every block to prevent spawns.
A slime spawned. However, in area where the ground was made from glass rather than covered with torches, slimes were not spawning, correctly.

What I expected to happen was...:

Mobs should not spawn in torches or carpet, possibly other similar items.

What actually happened was...:
Mobs did spawn.

Steps to Reproduce:

found in earlier description