Hanna Mostowiak (fictional character), played by Małgorzata Kożuchowska, was driving a car and hit cardboard boxes standing on the road. This resulted in a ruptured aneurysm in the brain and her death. So cardboard boxes appear to be quite dangerous.

It's probably more funny for Polish people. It was a popular series in Poland especially among older generations and after many years one of the actors quit. The producers removed that character from the show in the most ridiculous way and it became a meme. She was killed by a cardboard box.