Spiders can climb on a mob and then climb the edge of the world.
I thought the spider would just climb over the mob.
The spider did, but then kept climbing up the edge of the world.
Steps to reproduce:
1. Start a game in survival
2. Go to the edge of the world
3. Get a mob chasing you with a spider chasing you behind it.
4. Run along the edge of the world
5. The spider will now climb over the mob, but instead of falling back onto the ground, it will keep climbing up the edge of the world and eventually fall.