A beautiful poster created by Canadian designer Nelu Wolfensohn for A Celebration of Canadian Artists at Expo 86, found via the Canadian Design Resource.  Great color palette, typography and use of perspective and shade to create the impossible shape.

“The type’s optical illusion symbolizes an impossible reality, a different universe, proper to the field of arts.” – Nelu Wolfensohn

Check out Nelu Wolfensohn’s portfolio for more of his wonderful work. I apologize for the poor quality of the image, I wasn’t able to find a larger version online.