The trefoil knot also bounds a Möbius band.

When you first pull the trefoil from the soap, you see an object with four faces. One is in the middle and the other three span the three outer arcs. If you pop the middle face, you get the Möbius band above. If you pop one of the others, you get a object which doesn't have one of the outer arcs in its boundary. I was surprised this could happen.