What I expected to happen was...:
just stop and not climb

What actually happened was...:
Spiders climbed up the worldborder as I was testing the mobs in my Custom texture pack.

Steps to Reproduce:
1. Put a spider by the worldborder
2. let it sit there
3. look away and do something else for awhile
4. watch it climb up to height of 350
5. it will disappear until you look down and it will be at the top of your screen still climbing away