Norway issued a "Preserve the Polar Regions" sheet in February of 2009 and it consisted of a pair of 8 Kr stamps. One stamp had UV sensitive print that showed the ozone layer coverage of the Earth and the other was printed with a foil embossing technique. They look very nice. But what confused me was the perforations. They are perforated as square stamps but the globe graphic in each stamp is perforated as a circular stamp. However the 8 Kr and Norway labeling is not within the circular perforation boundary so I can't consider them circular stamps.
Here are some of my favorite recent circular issues: