A strange symbol writ large on the floor:

[A triangle with a stick into its back and a bobble on the opposite point, all in a circle for aesthetic effect.]

It is a symbol known only to initiates of the tightly-knit society called "the Known World's International Guild of Not-Tyers", a group of magicians, philosophers and logicians who are dedicated to unravelling (that is, not tying) intricately tangled problems of logic. The Grand Wizard of Many Things is an initiate of the fourth order (i.e. it's not true that he isn't not a not-member of the Not-Tyers), and the symbol appears here as an example of floored logic.