In 20w27a and below a mob (eg Zombie) would not pathfind onto a double carpet, for example a one block deep ditch covered with two carpets.  They would stop at the last block before the ditch.

In 20w28a at onwards the mob now pathfinds onto the carpet and then spins erratically until it changes the path finding target through either random walk or the target player moving blocks.  The mob then path finds off the carpet onto the other side of the carpet "barrier".

This makes carpet barriers at entrances no longer function as before .