Put the summary of the bug you're having here

What I expected to happen was...:
They should not be able to climb the boarders.

What actually happened was...:
They climbed the world boarders. :/

Steps to Reproduce:
1. Make it night so mobs start spawning
2. Spawn some spiders in
3. Make the world boarder pretty small.
4. Watch! They should sooner or later start climbing the walls(World Boarder)