Humanoid bugs

This is a list of known bugs with humanoids, containing detailed information about the bugs, their reproduction steps, and potential workarounds.

Setting Humanoid.HipHeight to 0 on R15 rigs

In any R15 rig, if you set Humanoid.HipHeight to 0, the humanoid will speed off in the direction it is facing when it makes contact with a surface, as if it were standing on a fast conveyor belt. This is due to the HumanoidRootPart touching the ground, which most likely causes a miscalculation in collisions between the HumanoidRootPart and the surface it is on.

Workarounds

There are no known workarounds at this time.

Reproduction
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local Players = game:GetService("Players")

Players.PlayerAdded:Connect(function(Player)
    Player.CharacterAdded:Connect(function(Character)
        Character:WaitForChild("Humanoid").HipHeight = 0
    end)
end)

Setting HumanoidRootPart.CollisionGroupId to a non-existant collision group

If HumanoidRootPart.CollisionGroupId is set to a collision group that does not exist, the humanoid will speed off in the direction it is facing when it makes contact with a surface, as if it were standing on a fast conveyor belt. This is due to Humanoid.HipHeight not being respected if the assigned group for HumanoidRootPart does not exist.

Note

While BasePart.CollisionGroupId can be set by developers, the recommended way to assign collision groups is via PhysicsService:SetPartCollisionGroup(), as noted the Roblox API documentation

Workarounds

There are no known workarounds at this time.

Reproduction
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local Players = game:GetService("Players")

Players.PlayerAdded:Connect(function(Player)
    Player.CharacterAdded:Connect(function(Character)
        Character:WaitForChild("HumanoidRootPart").CollisionGroupId = 1
    end)
end)