Designer's mistake
Where people work, mistakes are made, and the same applies to the design of stamps. So many stamps have been issued with design flaws or errors of other kinds that it has become a collecting field in itself. Here are a few examples of stamps where things didn't go quite right.
Spelling error in country name
In 1948, this Belgian stamp series was issued with the theme of the Abbey of Chèvremont. Unfortunately, the designer of this stamp series had placed the umlaut on the country name on the wrong vowel.
President with six fingers
In 1947, this series of stamps was issued in Monaco in honor of CIPEX in New York. When you study the 50c stamp more closely, you see that President Roosevelt's hand has no fewer than six fingers.
ratio search
In this series of stamps from the Netherlands from 1974, the drawn tennis ball is disproportionately much too small.
Space travel with a grain of salt
This stamp from 1965 features Voskhod 2 and the spacewalk of Russian cosmonauts. The image on the stamp is very imaginative for several reasons, primarily because the capsule door is open.
Life-threatening passenger bus
On the yellow bus of the 15 pfennig stamp, the passenger entrance is on the wrong side, meaning passengers would have to disembark in the middle of the street, since traffic drives on the right in Germany.
Snow crystals with branch too few
The Soviet Union issued this series of stamps featuring winter sports athletes in 1964 in connection with the Olympic Games in Innsbruck. The snowflake on the stamp has five branches. In reality, snowflakes always have six branches.
Faulty globe
Austria issued this series of three stamps in 2012. At first glance, there is nothing wrong with it, but if you take a closer look at the third stamp, you discover that the designer conveniently omitted Antarctica from the globe.
A hundred years too late
Macau issued this series of stamps in 1998 to commemorate the fact that the Portuguese explorer Vasco da Gama launched an expedition to India 500 years earlier, in 1498. However, the stamps state the wrong year, namely 1598 instead of 1498.
Fun topic!
A well-known design error is a GDR stamp from 1956. The stamp depicts composer Schumann, but with a Schubert composition in the background.
http://www.postbeeld.com/nl/stamps/view/sdd0528-r.-schumann-2v-with-wrong-music/
This one is also nice:
http://www.postbeeld.com/nl/stamps/view/sdma0707-diplomatic-relations-with-the-netherlands-1v/
The bottom flag is supposed to represent the Dutch flag!
Oh well, and many more can be thought of.
Incidentally, the bus could be one that drives on the left, hence the door is depicted on the correct side.
I believe this is called Errinophilia. I had a catalog of this (Yvert) which was unfortunately lost in water damage. There are so many examples: a French stamp featuring Descartes on which the title of his book is misspelled. A new stamp was issued for this. Italy: a road safety stamp with a traffic light depicted upside down (Green Top). But also the Netherlands: the 1956 Melbourne Olympic Games. The Netherlands boycotted those games due to the Soviet invasion of Hungary. This is also mentioned in that catalog. A Mongolia Olympic Games block in which 4 bobsledders have to squeeze themselves into a bobsled that has room for less than 2.
'@Per: Printing errors are not called Errinophilia. Errinophilia refers to commemorative stamps or sealing stamps. They look like postage stamps, but they are not, because they are not frankable. See the blog post from May 3, 2011 regarding this.
Well, look at that, Willem, and that is how we learn more about our hobby every day, isn't it?