I don't know if this was already posted but the monster in question(a badnik from sonic called Caterkiller) isn't climbing a single block like other monsters do. Here's an example since I don't have any kind of video editor. O's are a block and M is the monster. OOOO M OOOOOOOO As you can see the monster has a single block layer to climb but when it goes towards the wall it stops in place while still moving. Note: The monster USED to climb the layer when the game was in 1.2. Edit: This should be better. An image showing the monster stuck trying to climb the ledge.