Chickens can walk through the gap between a roundpole fence and a roundpole fence cornerpost.
I'm not sure if other creatures can do this.
As you can see, one chicken is inside my fenced in area, and another is in the process of walking into the fenced in area by the same means.
To recreate: Build two fenceposts. Build fences from one fencepost until you contact the other one. That contact point is where the bug occurs.
As I said, very minor, but I suppose it's worth noting either way.