The bug

If you have skeleton or another mob riding a horse for example, it will jump onto a block and start spinning repeatedly. I think this is because when the skeleton tried to go down a block it is using its own AI and position (thinking it is floating) instead of the horse's AI (which already is on the ground) so it starts spinning because they can't walk.

Implications

• Mobs riding for example a horse get stuck at block edges
  • Horse has saddle: Horse and rider start spinning
  • Horse has no saddle (for example skeleton trap): Horse and riding start wiggling
• (?) Mobs causing ridden mob to walk up blocks instead of going down because they think they are floating

How to reproduce

1. Summon a mob riding a horse

   /summon horse ~ ~ ~ {Tame:1b,Passengers:[{id:husk}]}
   

2. Summon a villager or switch to Survival mode and walk up some blocks