In the Netherlands, where Pat & Mat are known as Buurman & Buurman ("Neighbor & Neighbor"), the show is not silent, with both characters being voiced by Kees Prins and Siem van Leeuwen.
Under communist rule, someone objected that the colors of Pat & Mat shirts are making fun of tense Soviet-Chinese relationships, with Pat's yellow shirt representing China and Mat's red shirt the USSR. The authors were so forced to change Mat's shirt from red to a neutral gray until 1989, when the Velvet Revolution resulted in the collapse of Czechoslovakia's Communist regime and it was safe to make Mat's shirt red again.
Both animators Jan Keks and Karel Klos never actually existed. They are names Jan Klos put in the credits when he felt like his work wasn't as good as it could be.