Mobs (at least Zombies and Creepers) can't go from a carpet to a slab or a block. They stay on the carpet. This work also with snow layers (with at least 2 layers as the first doesn't change the height)
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Confirmed for unmodded 1.11.2
Relates to MC-13660
Confirmed for 1.13-pre2. Does not happen with carpets anymore, only with snow layers. I attached a new screenshot.
18w49a affecting full block, but not slab due to MC-134847
I found this on a spiggot server, but has the same comportement on a 1.11.2 single player game.