When its daytime, place down an empty spawner under the sun and set it to spawn spiders by using it with a spider spawn egg. Make sure there's no dark places around.

What I expected to happen was:
No spiders should be spawning

What it actually happened was:
Spiders start spawning 'inside the ground', which then begin to suffocate.

When you place the spawner in a dark area, some spiders spawn normally while some spawn inside blocks.

This also applies to cave spiders or silverfish.

As for other taller mobs (for example zombies), they can spawn inside a 1-block-high area.

You could see this inside spider dungeons, where even if you light up the entire dungeon with torches, spiders can spawn inside the walls.

In the screenshot, you could see that the spider that is newly spawned is suffocating inside the wall.