When pressed against a fenced corner, mobs think you are on the other side.

How to test - player

Make a pen (pic1). Have a mob, maybe a cow or a zombie (pic2), follow you and go inside the pen. When you go all the way into the corner they suddenly think you are outside and turns around and starts pathfinding to that position (pic3/pic4). By just moving inwards very slightly they turn around again and tracks you back into the pen.

I haven't tested with iron bars or walls.

Other mobs

This also happens to other mobs like turtles and axolotls. They get stuck on waterlogged fences and can't swim away unless you first push them away slightly. This is not only in corners. This is seen in pic5 where they are clearly attracted by the bucket but cannot swim away. Give them a slight push away from the fence and they start to follow you. As soon as any of them gets to close to the fence they are stuck again. Same with turtles.

I haven't tested with any other mobs.