Occasionally stamps are issued with design errors. Sometimes they are discovered before issuance and occasionally it leads to an issue being reprinted or not being issued at all. Most of the time however an emission simply appears, including the error or errors.
A very recent example are the PostNL public transport sheets issued on August 19, 2019. One of the stamps in the sheet with value “1 Netherlands” shows the Zwolle-Stadskanaal railway with an electrical train. This route never existed and to this day the railway line to Stadskanaal has no overhead wires. Instead this should have been stated to be the Zwolle-Emmen route, as the designer had actually devised in his original plan for this issue.
Both sheets show around the edge of the sheets names of places that were now or in the past accessible by public transport. Possibly there are more inaccuracies to discover with the very many name entries, but what is at least incorrect is that on sheet “1 Netherlands” the name “Stien” should have been Stiens. On sheet “1 International” Walburg is listed. This has never been a hamlet, village or place. Today this is a neighborhood in the municipality of Zwijndrecht, while in the past it was a so-called “rentmeesterwoning” of an agrarian.