I thought I was just lagging when I walked through that too. Since it looks like lag when it pushes you to the other side. But you can walk into it from the side.

The issue here is that the Spiral Knights client has a collision tile placed there while the servers do not. If you try walking through it, you'll see that your client thinks you'll collide there but a few seconds later, the server corrects you to your actual location as the server and other players see it.