Dan Brown’s ‘Angels and Demons‘, created a lot in interest in ambigrams. The ambigram has a design that presents several instances of words when rotated through a fixed angle. This is usually 180 degrees, but rotational ambigrams of other angles exist, for example 90 or 45 degrees.
Suck UK design, has this innovative ‘Do not Disturb’ door knob hanger for sale. The words in the center, change in meaning, when you turn the door knob hanger upside down.