Here's one. In 1945, a joint Soviet-Polish commission is establishing a new frontier. There is a farm right on the line.
They decide to ask the farmer for his preference. He replies that he wants to be in Poland, because in Russia it's so cold in the winter.
When I was first told this joke (in London) it was clearly meant to show that Poles are stupid. Later, in Poland, I tried it on my interpreter, a passionately patriotic Polish woman.
She beamed and said: "There you are - we Poles are so cunning, we can always make rings round the Russians."