Blivet
The blivet is an undecidable figure, an optical illusion and an impossible object.

An anonymously-contributed version described as a "hole location gauge" was printed in the June 1964 issue of Analog Science Fact/Science Fiction, with the comment that "this outrageous piece of draughtsmanship evidently escaped from the Finagle & Diddle Engineering Works".
It is also called a "two-pronged trident" and a "three-legged widget".