The Bug

Discovered while testing MC-273971.

Mobs do not get inside minecarts that are not placed on top of rails. They seem to be unable to get in the minecart at all. However, they can get in a minecart on top of rails, and then the rail can be broken, or the minecart can be pushed off the rail, so that the mob is in the minecart while it is not on top of a rail.

If the "Minecart Improvements" experiment is enabled, this bug does not occur. 

Steps to Reproduce

1. Ensure you are on a world with the "Minecart Improvements" experiment disabled.
2. Place a minecart on a rail, and then break the rail so that the minecart is on a block.
3. Place a mob that can get in a minecart near the minecart, such as a creeper.
4. Push the mob into the minecart.

Observed Results

The mob does not enter the minecart.

Expected Results

The mob enters the minecart.