The bug
In snapshot 23w41a, carts where changed somehow so mobs that dismount carts now can be placed inside of blocks when before they needed air at the location.
Also players can clip through the floor using this.
A video of this issue can be found here.
How to reproduce
Place and power an activator rail such that there are no valid surrounding blocks upon which the player can be placed
Attempt to enter the minecart
❌ The player may be placed within the block below the minecart
Linked issues
is duplicated by 2
relates to 2
Comments 5
Can confirm in 1.21 pre-1 and 1.21 pre-2,
This issue is very important cus it breaks all vehicle-passenger related contraption that technical minecraft community has designed over the years, and it breaks most of them fundamentally cus most of them rely on the cart dismounting its passenger specifically not on a rail block, as it is very hard to get a mob from a rail in a controllable manner (without causing entity cramming in entity holding chambers etc).
Expected behavior is for entities to dismount the carts to the side of the cart, exactly the way it was in 1.20 and in versions prior to that
The to replicate got changed to something else not related to this original bug report. See here to reproduce. MC-272060
Players clipping through the floor is covered by MC-263712