Spider stuck to cave ceiling

What I expected to happen was...:
I was caving and encountered a spider. I expected the spider to engage me in combat.

What actually happened was...:
The spider stayed stuck to the ceiling, his or her eyes following me around the room but never moving. Just making spider screeches.

Steps to Reproduce:
1. Go into a cave
2. Encounter a spider
3. Become disappointed when it doesn't fight you, or move.

I was unable to duplicate this myself but... it happened.