Monsters fall through cornerless walls

When I build a mob-grinder shaft cutting through a floor, and the shaft doesn't include the corner columns, a monster sometimes falls through the corner and lands on the floor, even though there's no width for it to fall through. This means it must be able to fall through just a vertical line with no width, and I'm pretty sure that's wrong. NB: So far I haven't seen this happen with a mob that survived the fall.