Here is a real life example of "Schröder's staircase" illusion. This optical illusion is one of the more famous, classic reversible figures. Can you see the stairs right side up and upside down?
Up the Down Staircase
Flickr Artist: theentiremikey
Here is the classic version of the illusion.
Looking at the illusion above, if the green wall is closest to you then you will be walking upside down. If the pink wall is closest to you then you are walking upright. Either way you are walking on the same blue treads. Of course if your willing to walk sideways then you can use the black treads.