The bug
Mobs cannot pathfind properly and thus move very quickly on blocks without a solid top face, especially if there is air or another block without a solid top face below.
This is known to affect the following blocks (list may be incomplete):
Bottom slabs
Farmland
Dirt paths
(Empty) end portal frames
Enchanting tables
Carpets above string
Chests
Campfires
It seems to be affecting all mobs, though it seems like the issue is best reproduced with skeletons due to their strafing.
How to reproduce
Create a "Redstone Ready" flatland world
Run (optional, but makes the issue easier to see)
/fill ~ ~-2 ~ ~20 ~-12 ~20 airRun
/fill ~ ~-1 ~ ~20 ~-1 ~20 oak_slab(Or any other affected block)
Give yourself some skeleton spawn eggs
Switch into survival mode
Spawn a skeleton on top of the slabs
→ ❌ The skeleton moves really quickly
Example
Here is an example in video: https://youtu.be/rNsKKNCFxMM
Related issues
is duplicated by
MC-159890
Skeleton Moving faster on tilled land
MC-161232
Skeletons goes incredibly fast on slabs
MC-171915
skeletons are broken on grass path
MC-171955
Skeleton moves '1000'Mph on half slabs
MC-174789
Skeletons on slabs sometimes can't hurt the player when they're close enough to them
MC-176197
Skeletons move very quickly on half slabs
MC-181253
Skeleton Broken Agility Exists in 1.15.2
MC-188283
Villager pathfinding is broken on double carpets
MC-188964
Some entities (such as villagers) get stuck on waterlogged bottom slabs
MC-189931
Sksleton Runs Fast on Tilled Soil
MC-194628
Pathfinding for almost all mobs break when on carpets with a non-solid block underneath
MC-194632
Mobs are unable to pathfind on top of (extingushed) campfires
MC-204326
Villager(s) and Pathfinding
MC-212595
Villager spins in circle when standing on carpet above water
MC-233843
Axolotls get stuck on extinguished campfires
MC-261572
Mob ai
relates to
Attachments
Comments
migrated
violine1101
Can confirm. A similar effect occurs with waterlogged top slabs, see MC-156022.
migrated
Confirmed on 19w38b
migrated
Affects 20w12a
migrated
Bedrock edition also has this bug MCPE-47075
migrated
Villagers are spinning on the spot on waterlogged top half slabs in 1.15.2
migrated
I have noticed some pathfinding changes in regards to carpet on top of string/torches/carpet. Usually it totally inhibits mobpathfinding but as of 1.16.2 mobs are able to walk around on this setup, despite their pathfinding still being confused leading them to spin around from time to time, they are still capable of escaping from this setup.
migrated
Yes, it appears the new pathfinding attributes addedin 1.16.2 to mobs and villagers now allows them to pathfind over double carpet stacks or other air block methods. Not sure if the pathfinding work was intended to 'fix' this mechanic or not, but now mobs on double carpets can pathfind away, and can behave strangely when on top of carpets with air blocks below them. (spinning in circles). I kind of wish they kept it the old way, as this was a very simple and compact method for both mob proofing and handling villagers (trading halls, etc).
migrated
They move normally for me.
ampolive
Can confirm in 1.17. Mobs sometimes just don't move on top of double carpets, and stay completely still on unlit campfires.
ampolive
Can confirm in 1.17.1 Pre-release 1.
ampolive
Can confirm in 1.17.1.
migrated
version: 1.21 ...
mob still freeze when in slabs with a space of 2 blocks between 2 half blocks, slab on bottom and top
Minecraft386882
Can confirm for 1.21.3
Minecraft386882
Confirmed in 24w46a.