At first glance it looks like a doorknob.
Many optical illusions go undetected for the same reason.
32 is sort of a nice one. It doesn't look especially like anything until you are told what it is supposed to be. In this case a long tunnel with the night sky beyond. This kind of optical illusion is based upon someone else's misperception, and is the basis of all Art. Illusions like this can be interpreted in all sorts of other ways, but they would be wrong.