The bug
Mobs ignore open trapdoors and act like they are not open. This causes mobs to fall into places they would not usually end up like dying when falling off a > 24-meter opening space, which will kill them, and often traps them in weird places.
Related issues
is duplicated by
relates to
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Trapdoors are considered a solid block by every mob. So they think they can't fall trough even though the trapdoor is open.

Is this still an issue in the current Minecraft Snapshot 15w46a or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
This issue relates to MC-94115
Confirmed for 16w04a
Is this still an issue in 16w05b?
Confirmed for 16w06a and 16w05b
Confirmed for 1.9-pre1
Confirmed for 1.9-pre4 and 1.9-pre3
Confirmed for 1.9.1-pre1
Can Confirm for 1.9.1 pre-release 3
Confirmed for 16w14a
Confirmed for 1.9.3-pre1
Confirmed for 1.9.3-pre2
Confirmed for 1.9.3-pre3
Confirmed for 16w21a.
Confirmed for 16w21b.
Confirmed for 1.10-pre1.
Confirmed for 1.10-pre2.
Confirmed for 1.10.
Confirmed for 1.10.1.
Confirmed for 16w39c
" Trapdoors are considered a solid block by every mob. So they think they can't fall trough even though the trapdoor is open. "
And in the case of a 1x1 hole with a trapdoor over it they are right! Cows, sheep, and pigs which will fall through a 1x1 hole will not fall through that same hole if it is covered by an open trap door.

Confirmed for 17w18b
Confirmed for 1.13 Pre-release 8. This behavior does not happen with the new wooden trapdoors in 1.13. See MC-132358
Confirmed for 1.13.1.
A lot of farms rely on this bug such as my zombie pigman farm as I need them to walk off the edge using trap doors, I think this should stay as it is due to how many farms use trapdoors to get mobs to walk off the edge towards the kill chamber.
as @SuperHotC4 mentions above, I also think this is a kind of bug that shall be turned into a feature unless minecraft devs creates a block for this behavior, just check how many farm tutorials over youtube are relying on this bug/feature
Confirmed in release 1.14.1
I don't get it. If it still works, then why did they make those ''empty flowerbeds/water troughs/barrels/baths'' made out of spruce trapdoors in 1.14 Taiga villages. It is very annoying, if I see something like this, I always break it, no matter what.
Confirmed for 20w09a
Wait so this has been an issue since 2014 (6 Years) and is reported multiple times every year and this is still an issue? Why do worlds still naturally generate with trap doors in villages like this if there is no plan to fix it??????????
Affects 20w10a, can I request ownership?
Affects 20w11a and 1.15.2
I think this is more of a feature than a bug cause its a trap door and the mob isn't suppose to expect it.

upvote to @awesome90 's comment hahah
@awesome90, this is definitely not an intentional feature.

In 20w12a
Confirmed for 20w13b. A bug over 6 years old
Confirmed for 20w15a
This bug gets used in many mob farms. If this gets fixed it would break them.
Confirmed for 20w16a

In 20w18a

In 20w19a
Don't break mob farms!
There is no current replacement, so it shouldn't be removed, similar to tnt duping, if they add a tricker block or something, then it's fine to be removed, but for now, it has to stay.
Well, I think that tricking mobs shouldn't be removed, but mobs getting trapped inside them should definitely be fixed. It's pretty annoying
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working as intended

can you provide official proof of this being intended?

When commenting that you need to include a official source

In 1.16 Pre 3
In 1.16rc1
"working as intended"?? How is villagers, cats and probably more being trapped in the villages trapdoor decoration thing intended? Mobs not being able to jump over trap doors definitely isn't intended. It ruined my village by trapping them so I had to destroy the trap door decoration thing.
For these trapdoors still generating naturally - MC-199420.
A smart solution for the Village generation problem would be if Mojang addresses MC-130584, because from my experience mobs try to avoid Trapdoors with blocks or fluids in the inside my solution was always fill these decorations with Water so they either avoid these or just float on top of the water, the problem is when MC-130584 takes place, this issue makes problematic to save these Decorations Structures with Data Packs because the water will spread & I believe these decorations were previously introduced with water inside during the snapshots of 1.14, later on the water was taken away, hopefully now Mojang decides to finally fix this problem and bring back the water in these decorations so Villages no longer generate with traps inside.
Another fix could be that mobs could autojump if there is a 1-block high obstacle in their pathfinding.
Farms have been using this for ages now, fixing it would be a terrible mistake.
@unknown, for missing water see MC-205894
Vanilla 1.19.4 never fixed
[media]So we should babysit every villager in game to make sure they don't die?
This bug gets my pet wolves killed so often when trapdoors are used to decorate bridges and cliffside paths.
If this is intended then why do villages generate with death traps that the villlagers walk into and get stuck in?