Graeme −> Odds and Ends −> Tricks −> Poiuyt a.k.a Blivet, Devil's Fork 

Three perspective illusions: upper right is the poiuyt (a word spelled by running the fingers across the typewriter keyboard, also known as the blivet or Devil's Fork); left is the Penrose Frame, shown with three perspective-impaired hexnuts (enlarged at the lower right).

Internet references

Michaelís Optical Illusions & Visual Phenomena: Impossible Objects